f^-1 ( x) = y means that f(y) = x, so we know that f(-2) = 3.
Compute f(-2) = k(2 - - 2 - (-2)^3) = k(2 + 2 + 8) = 12k
So 12k = 3
k = 3/12 = 1/4
Compute f(-2) = k(2 - - 2 - (-2)^3) = k(2 + 2 + 8) = 12k
So 12k = 3
k = 3/12 = 1/4
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f(-2) = 3 ---> k = ...