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Curve Fitter 2

Can you construct a cubic equation with a certain distance between its turning points?

Curve Fitter

Age 16 to 18 Challenge Level:

The following function fits exactly, but has the wrong asymptotic behaviour. $$y= 0.75x^4-2.5 x^3+0.75x^2+3x$$

The following cubic nearly fits. $$2 y = x^2(2x-9)+11x$$

This following quintic exactly fits. $$4y = 4x^5-21x^4+42x^3-45x^2+28x$$

The differential set to zero gives $$ 20x^4-84x^3+126x^2-90x+28 = (x-1)(x-2)(20x^2-24x+14)=0\;.$$

This is by choosing the coefficient of $x^5$ to be $1$.

This curve only has 2 real turning points.

You can see these graphs on the attached xls.