Basics of Matrices
Matrices are just tables of numbers (or variables) listed in rows and columns, exactly like a table. For example,
\(\displaystyle{ A = \left[ \begin{array}{ccc} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \\ a & b & c \end{array} \right] }\)
This matrix is a 4x3, with 4 rows and 3 columns. The matrix is called A.
To delineate a matrix, it will usually be written with brackets, like we've done here, or with parentheses. However, we never use straight vertical lines unless we are talking about a determinant.
The main idea with matrices is that not only is the number/variable important, but also the position it occupies. For example, matrices C and D below are not equal, even though they are the same size (2x2) and have the same elements, because some of the numbers are in different positions.
\(\displaystyle{ C = \left[ \begin{array}{cc} 1 & 2 \\ 3 & 4 \end{array} \right] }\) 

\(\displaystyle{ D = \left[ \begin{array}{cc} 2 & 1 \\ 3 & 4 \end{array} \right] }\) 
So for two matrices to be equal, the dimensions must be the same and the numbers/variables must be in the same position.
If you are just starting to learn about matrices, these first two videos are for you.
video by Khan Academy 

This second video explains matrices and some basic operations in more depth with plenty of examples.
video by MIP4U 

Overview of Basic Matrix Operations
Addition   As described in the last video above, matrix addition requires both matrices to have the same number of rows and columns. We just add the corresponding elements.
Scalar Multiplication   Multiplication of a matrix by a scalar just involves multiplying each element by that scalar.
Subtraction   Finally, matrix subtraction is just a combination of addition and multiplication by the scalar \(1\).
If any of these operations is difficult for you, it will help to go back and watch the last video above and then work some practice problems.
Matrix Multiplication
Unlike with addition, multiplication of two matrices is quite involved and the technique is somewhat counterintuitive. Here is a good introduction video and example. He explains quite thoroughly how to do matrix multiplication. [More discussion and practice problems on multiplication of matrices can be found on the matrix inverses page.]
video by Khan Academy 

Matrix Division
The next natural question is, what about matrix division? Well, matrix division is even more complicated that matrix multiplication. Let's look at an analogy with numbers.
An Analogy   If we want to divide 2 into 3, i.e. 3/2, we can think about it this way. We find the (multiplicative) inverse of 2, written either \(1/2\) or \(2^{1}\) and then multiply this inverse to 3, i.e. \(3/2=3(2^{1})\).
That is how we do matrix division. We need to find the multiplicative inverse (usually just called the inverse when talking about matrices) of a matrix and then multiply. Finding the inverse is quite involved and we will discuss this on a separate page.
Okay, now you are ready for some practice problems.
Practice
Instructions  Unless otherwise instructed, perform the indicated operations on these matrices.
Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 3 & 1 \\ 2 & 0 \end{array} \right] + \left[\begin{array}{rr} 7 & 2 \\ 3 & 5 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 3 & 1 \\ 2 & 0 \end{array} \right] + \left[\begin{array}{rr} 7 & 2 \\ 3 & 5 \end{array} \right] }\)
Solution 

video by Khan Academy 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 3 & 1 \\ 2 & 0 \end{array} \right]  \left[\begin{array}{rr} 7 & 2 \\ 3 & 5 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 3 & 1 \\ 2 & 0 \end{array} \right]  \left[\begin{array}{rr} 7 & 2 \\ 3 & 5 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rrr} 1 & 7 & 5 \\ 0 & 3 & 10 \end{array} \right] + \left[\begin{array}{rrr} 5 & 0 & 3 \\ 11 & 1 & 7 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rrr} 1 & 7 & 5 \\ 0 & 3 & 10 \end{array} \right] + \left[\begin{array}{rrr} 5 & 0 & 3 \\ 11 & 1 & 7 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 0 & 1 \\ 3 & 2 \end{array} \right]  \left[\begin{array}{rr} 1 & 3 \\ 0 & 5 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 0 & 1 \\ 3 & 2 \end{array} \right]  \left[\begin{array}{rr} 1 & 3 \\ 0 & 5 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 1 & 2 \\ 3 & 8 \end{array} \right] + \left[\begin{array}{rr} 10 & 0.6 \\ 0.4 & 14 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 1 & 2 \\ 3 & 8 \end{array} \right] + \left[\begin{array}{rr} 10 & 0.6 \\ 0.4 & 14 \end{array} \right] }\)
Solution 

video by PatrickJMT 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rrr} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \end{array} \right] + \left[\begin{array}{rrr} 10 & 20 & 30 \\ 40 & 50 & 60 \\ 70 & 80 & 90 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rrr} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \end{array} \right] + \left[\begin{array}{rrr} 10 & 20 & 30 \\ 40 & 50 & 60 \\ 70 & 80 & 90 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 2 & 3 \\ 4 & 5 \\ 6 & 7 \end{array} \right] + \left[\begin{array}{rr} 1 & 5 \\ 8 & 9 \\ 3 & 4 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{\left[\begin{array}{rr} 2 & 3 \\ 4 & 5 \\ 6 & 7 \end{array} \right] + \left[\begin{array}{rr} 1 & 5 \\ 8 & 9 \\ 3 & 4 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{ \left[\begin{array}{rr} 1 & 2 \\ 3 & 8 \end{array} \right]  \left[\begin{array}{rr} 10 & 0.6 \\ 0.4 & 14 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{ \left[\begin{array}{rr} 1 & 2 \\ 3 & 8 \end{array} \right]  \left[\begin{array}{rr} 10 & 0.6 \\ 0.4 & 14 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{10 \left[\begin{array}{rr} 2 & 3 \\ 4 & 5 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{10 \left[\begin{array}{rr} 2 & 3 \\ 4 & 5 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{5 \left[\begin{array}{rrr} 1 & 2 & 8 \\ 3 & 7 & 1 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{5 \left[\begin{array}{rrr} 1 & 2 & 8 \\ 3 & 7 & 1 \end{array} \right] }\)
Solution 

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Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{4 \left[\begin{array}{r} 4 \\ 7 \end{array} \right] + 2 \left[\begin{array}{r} 0 \\ 3 \end{array} \right] }\)
Problem Statement 

Unless otherwise instructed, perform the indicated operations on these matrices.
\(\displaystyle{4 \left[\begin{array}{r} 4 \\ 7 \end{array} \right] + 2 \left[\begin{array}{r} 0 \\ 3 \end{array} \right] }\)
Solution 

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Instructions  Unless otherwise instructed, use the matrices \(S\) and \(T\) in these problems.
\(\displaystyle{ S = \left[ \begin{array}{rr} 4 & 7 \\ 2 & 1 \end{array} \right] }\)
\(\displaystyle{ T = \left[ \begin{array}{rr} 3 & 7 \\ 5 & 6 \end{array} \right] }\)
Unless otherwise instructed, calculate \( S+T \), where
\(\displaystyle{S = \left[\begin{array}{rr} 4 & 7 \\ 2 & 1 \end{array} \right] }\)  \(\displaystyle{T = \left[\begin{array}{rr} 3 & 7 \\ 5 & 6 \end{array} \right] }\) 

Problem Statement 

Unless otherwise instructed, calculate \( S+T \), where
\(\displaystyle{S = \left[\begin{array}{rr} 4 & 7 \\ 2 & 1 \end{array} \right] }\)  \(\displaystyle{T = \left[\begin{array}{rr} 3 & 7 \\ 5 & 6 \end{array} \right] }\) 

Solution 

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Unless otherwise instructed, calculate \( TS \), where
\(\displaystyle{S = \left[\begin{array}{rr} 4 & 7 \\ 2 & 1 \end{array} \right] }\)  \(\displaystyle{T = \left[\begin{array}{rr} 3 & 7 \\ 5 & 6 \end{array} \right] }\) 

Problem Statement 

Unless otherwise instructed, calculate \( TS \), where
\(\displaystyle{S = \left[\begin{array}{rr} 4 & 7 \\ 2 & 1 \end{array} \right] }\)  \(\displaystyle{T = \left[\begin{array}{rr} 3 & 7 \\ 5 & 6 \end{array} \right] }\) 

Solution 

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Unless otherwise instructed, calculate \( T3S \), where
\(\displaystyle{S = \left[\begin{array}{rr} 4 & 7 \\ 2 & 1 \end{array} \right] }\)  \(\displaystyle{T = \left[\begin{array}{rr} 3 & 7 \\ 5 & 6 \end{array} \right] }\) 

Problem Statement 

Unless otherwise instructed, calculate \( T3S \), where
\(\displaystyle{S = \left[\begin{array}{rr} 4 & 7 \\ 2 & 1 \end{array} \right] }\)  \(\displaystyle{T = \left[\begin{array}{rr} 3 & 7 \\ 5 & 6 \end{array} \right] }\) 

Solution 

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