`x^3-4x^2=-4x`
`<=>x^3-4x^2+4x=0`
`<=>x(x^2-4x+4)=0`
`<=>x(x-2)^2=0`
`<=>` $\left[\begin{matrix} x=0\\ (x-2)^2=0\end{matrix}\right.$
`<=>` $\left[\begin{matrix} x=0\\ x-2=0\end{matrix}\right.$
`<=>` $\left[\begin{matrix} x=0\\ x=2\end{matrix}\right.$
Vậy `S={0;2}`