How would I work this maths problem out?

The area for the whole roof (with the lift etc) is just its length times width. One side is (X + 5) and the other is (X + 8), so the area of the whole roof is (X+5)(X+8) = X^2 + 13X + 40. The area of the lift and stuff is 8*5 = 40, so you take that away since you don't want to include it in the area of the garden. The area of the garden is X^2 + 13X. If you want this area to be 126.75, then you want X^2 + 13X = 126.75.