I need help with a cal 2 question. They want me to show that f(g(x))=x for each x in the interval [-9, +inf.)?

Let f(x) = x^2 - 4x - 5 with D(f) = [2,+infinity) and R(f) = [-9,+infinity). Let g(x) = square root(x+9) +2 with D(g) =[-9,+infinity) and R(g) = [2,+infinity). Then g is the inverse of f. Show that, indeed, f(g(x)) = x for each x in the interval [-9,+infinity).