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 compute A - B, subtract the corresponding element of B from each element of A.
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 confusing.
Mentally fill in the two question marks.