Hope it helps

fg(x) means that i am actually putting the function g into the function f, which means the x in the f(x) is actually g(x) as for the composite fg to exist, the range of g is a subset of the domain of f, so what u need to do is to sub x = g(x) into the f function, then expand everything out and compare the elements. I am quite certain that this is correct as 5h - 5 = 5, which matches the constant in the given equation. This actually covers a little bit of jc math so i would be curious as to how is your teacher going to explain to your class.