Please help on this algebra!

What adds to be the bottom number but also multiplies to be the top number with 8 on the bottom and 15 on top

If f(x) = 4x - 3 and g(x) = x + 4, find (f - g) (x)

Sanderson is having trouble with his assignment. his shown work is as follows: − 3 7 7 3 = − 3 7 × 7 3 = − 21 21 = −1 however, his answer does not match the answer that his teacher gives him. complete the description of sanderson's mistake. find the correct answer.