Given f(x)=−2x^3 -3x + 1, the value of the derivative of the inverse function, f^−1(x) at x=−21 is (f^1)′(−21)= ?

Question

please explain how to solve this

Answer 1

Since f(2) = -21,
f^-1(-21) = 2
f'(x) = -6x^2-3

(f^-1)'(-21) = 1/f'(f^-1(-21)) = 1/f'(2) = -1/27