f(x)=(k-x)^2\,e^{-x}

k = -1.08 (slider from -3 to 3, step 0.01)
f(x) = (k - x)^(2.0) * exp(-1.0 * x)
f'(x) = f'(x)
A = Intersect(f', xAxis, (0.9199999999999399, 4.773959005888173E-14))
a = Line(A, yAxis)
B = Intersect(f, a, (0.9199999999999399, 1.594076164338057))
Text1 = Text("k animieren.")