Question

Seven less than a number is fifteen

Answer 1

7-x=15

7-(-8)=15

what we have to do in order to solve this is change the 2 subtraction signs into a addition sign because a negative plus a negative makes a positive.

Answer 2

Actually, its the other way around, its not 7-x=15 but
$x - 7 = 15 \\ x = 15 + 7 \\ x = 22$