Help on functions please.

A) Sub g(x) and h(x) into the equation:
x^2 + 3x - 40 - (-x -3)
= x^2 + 3x - 40 +x + 3
= x^2 + 4x - 37
Find x:
x= 4.40 and x= -8.40
b) Sub f(x) and g(x) into the equation: 
(x^2 - 64) / (x^2 + 3x -40)
= ((x+8) x (x-8)) / ((x+8) x (x-5))
= (x - 8) / (x-5)
Find x:
x= 8 and x = 5
How to get those number:
(x^2 - 64) = x^2 - 8^2 = (x-8) x (x+8)
( x^2 + 3x - 40) = (x^2 + +8x - 5x - 40) = x( x + 8) - 5( x + 8) = (x-5) x (x+8)

Try to do c and d :)