$x³ +3x² -6x -8 = 0$
$⇔ x².(x +1) +2x.(x +1) -8.(x +1) = 0$
$⇔ (x +1).(x² +2x -8) = 0$
$⇔ (x +1).[(x +1)² -9] = 0$
$⇔ (x +1).(x +1 -3).(x +1 +3) = 0$
$⇔ (x +1).(x -2).(x +4) = 0$
$⇔ \left[ \begin{array}{l}x +1=0\\x -2=0\\x +4 = 0\end{array} \right. ⇔ \left[ \begin{array}{l}x=-1\\x=2\\x = -4\end{array} \right.$
$Vậy$ $S =$ {$-1; 2; -4$}
