$ x² +(x+3)(5x-7) = 9$
$ ⇔ x² +5x² +8x -21 = 9$
$ ⇔ 6x² +8x -30 = 0$
$ ⇔ 6x² +18x -10x -30 = 0$
$ ⇔ 6x.(x +3) -10.(x +3) = 0$
$ ⇔ (x +3).(6x -10) = 0$
$ ⇔ \left[ \begin{array}{l}x +3=0\\6x -10=0\end{array} \right.$
$ ⇔ \left[ \begin{array}{l}x=-3\\x=\frac{5}{3}\end{array} \right.$
$ Vậy$ $ S =$ {-3; $\frac{5}{3}$}