The factors of an expression are the terms that are multiplying someone.
For example, in the expression:

The factors are 5 and 3 (5 is the factor of 'a' and 3 is the factor of 'b').
So in our expression, we have the number 3 multiplying the variable 'a' and the number -1 multiplying the variable 'b'. The number 7 is alone, so it has no factors.
Therefore the answer is:
3 and a, -1 and b.