Matrix multiplication can be quite confusing for many students and although the process seems pretty straight forward, it can be easily forgotten.
Before going into the details, we need to know that matrices are described by their rows followed by their columns. For example, a matrix with 2 rows and 2 columns is described as a 2 X 2 matrix.
Not all matrices can be multiplied together. When we multiply say a 2 X 2 matrix with a 2 X 1 matrix, notice that the 2 inside numbers are both 2, as seen below (boxed up). This tells us that they can be multiplied. The resulting matrix would be a 2 X 1 matrix, which is taken from the 2 outside numbers.
Once we have confirmed that multiplication can be done, we need to follow a process to work it out. This is where it can get confusing for some.
One of my students came up with a creative method to remember the process;
Arrows were drawn as seen above to remind us that we need to go across and down.