Which of the following is not a root of f ( x ) = x 3 - 11 x 2 + 36 x - 36 ?

The following is not a root of $x^3-11x^2+36x-36=0$

$\Rightarrow x^3-2x^2-9x^2+18x+18x-36=0$

$\Rightarrow x^2(x-2)-9x(x-2)+18(x-2)=0$

$\Rightarrow (x-2)(x^2-9x+18)=0$

$\Rightarrow x=2, x=\frac{9\pm \sqrt{81-72}}{2}$

$\Rightarrow x=2, x=\frac{9\pm 3}{2}$

$\Rightarrow x=2, x=6,3$

$\Rightarrow x=2,3,6$ are roots

$\Rightarrow x=4$ is not a root.