Verify lagranges mean value theorem for the function `f(x)=(x-3)(x-6)(x-9) on [3, 5]`

Explanation

`f(x) = (x-3)(x-6)(x-9)`

`= (x-3)(x^2 - 15x + 54)`

`= x^3 - 18x^2 + 99x - 162`

`f c in (3,5) st f'(c) = (f(5) - f(3))/(5-3)`

`f(5) = (5-3) (5-6)(5-9)`

`= 2(-1)(-4) = -8`

`f(3) = (3-3)(3-6)(3-9) = 0`

`f'(c) = (8-0)/2 = 4`

...