`x^2-(x+3)(2x-11)=9`
`<=> x^2-(2x^2-11x+6x-33)=9`
`<=> x^2-2x^2+5x+33=9`
`<=> -x^2+5x+33=9`
`<=> x^2-5x-24=0`
`<=> x^2-8x+3x-24=0`
`<=> (x-8)(x+3)=0`
`<=>`\(\left[ \begin{array}{l}x-8=0\\x+3=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=8\\x=-3\end{array} \right.\)
Vậy `S={8;-3}`