k = 1.3000000000000007 (slider from -5 to 5, step 0.1)
f_k(x) = 2 x^(3) - 3 k x^(2) + k^(3)
f_{-k}(x) = 2 x^(3) - 3 (-k) x^(2) + (-k)^(3)
W = TurningPoint(f_k)