Tentukan nilai f jika: f'(x) (invers) = 3 akar x+1

Di inverskan lagi
f-¹(x) = 3√(x+1)
y = 3√(x+1)
y/3 = √(x+1)
(y/3)²= x+1
x = (y/3)²-1
f(x) = (x²/9) – 1