You can think in reverse. Addition is the opposite of subtraction. So to solve 15-8, you can ask "8 plus what number is equal to 15?" and note how 8+7 = 15. So that's why 15-8 = 7. Similarly, 15-7 = 8 can be solved the same way.
Calculate either in your head or on a paper what plus 8 equals 15, you can solve it easily this way. It is probably easier to do in your head (9, 10, 11, ...), continue on and make sure to remember how much is the sum.
