Calculus
posted by Ann .
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)= ?
please explain how to solve this

Since f(2) = 21,
f^1(21) = 2
f'(x) = 6x^23
(f^1)'(21) = 1/f'(f^1(21)) = 1/f'(2) = 1/27