~ Bạn tham khảo ~
`x^3+9x^2-108=0`
`<=> x^3+12x^2+36x-3x^2-36x-108=0`
`<=> x(x^2+12x+36)-3(x^2+12x+36)=0`
`<=> (x^2+12x+36)(x-3)=0`
`<=> (x^2+2*6*x+6^2)(x-3)=0`
`<=> (x-3)(x+6)^2=0`
\(⇔\left[ \begin{array}{l}x-3=0\\x+6=0\end{array} \right.\)
\(⇔\left[ \begin{array}{l}x=3\\x=-6\end{array} \right.\)
Vậy `S={3;-6}`