(9x² -4)(x+1)=(3x+2)(x² -1)
⇔ (3x -2).(3x +2).(x +1) = (3x +2).(x -1).(x +1)
⇔ (3x -2).(3x +2).(x +1) - (3x +2).(x -1).(x +1) = 0
⇔ (3x +2).(x +1).(3x -2 -x +1) = 0
⇔ (3x +2).(x +1).(2x -1) = 0
⇔ 3x +2 = 0 hoặc x +1 = 0 hoặc 2x -1 = 0
⇔ x = $\frac{-2}{3}$ hoặc x = -1 hoặc x = $\frac{1}{2}$
Vậy S= {$\frac{-2}{3}$ ; -1 ; $\frac{1}{2}$ }