Answer:
(a) (f o g)(x) = x^2 - 15x + 54
(b) (g o f)(x) = x^2 + 3x - 9
(c) (f o f)(x) = x^4 + 6x^3 + 12x^2 + 9x
(d) (g o g)(x) = x - 18
Explanation:
f(x) = x^2 + 3x
g(x) = x - 9
(a)
(f o g)(x) = f(g(x)) = (g(x))^2 + 3(g(x)) = (x - 9)^2 + 3(x - 9)
(f o g)(x) = x^2 - 18x + 81 + 3x - 27
(f o g)(x) = x^2 - 15x + 54
(b)
(g o f)(x) = g(f(x)) = f(x) - 9 = x^2 + 3x - 9
(c)
(f o f)(x) = f(f(x)) = (x^2 + 3x)^2 + 3(x^2 + 3x)
(f o f)(x) = x^4 + 6x^3 + 9x^2 + 3x^2 + 9x
(f o f)(x) = x^4 + 6x^3 + 12x^2 + 9x
(d)
(g o g)(x) = g(g(x)) = x - 9 - 9 = x - 18