Step-by-step explanation:
Knowing that f(x) = 2x^2 - x - 4 and f^2(x) means f(f(x)), we get:
f^2(x) = f(f(x)) = 2(2x^2 - x - 4)^2 - (2x^2 - x - 4) - 4.....
You have to square the whole thing... You'll get a 4th degree equation. I know it's a bit boring .... but whatever.
