Question

How do you do 7-12? How do you do 13-18?

Answer 1

7-12 is like a negative number so it will be -5. And 13-18 would be -5 also. 

Answer 2

What you need to do is take 7-12 and 13-18 and flip them and add a negative sign to your answer .
So 7-12= -5 and 13-18= -5