It seems like subtraction could be defined as adding a negation of a matrix.
If A and B have the same number of rows and columns, then A - B is defined as A + (-B). Usually you think of this as:
To form A - B, from each element of A subtract the corresponding element of B.
Here is a partly finished example:
Notice in particular the elements in the first row of the answer. The way the result was calculated for the elements in row 1 column 2 is sometimes confusion.