9x² - 3 = ( 3x + 1 ).( 2x -3 )
⇔ 9x² -3 - ( 3x + 1 ).( 2x -3 ) = 0
⇔ 9x² -3 - (6x² -9x +2x -3) = 0
⇔ 9x² -3 -6x² +7x +3 = 0
⇔ 3x² +7x = 0
⇔ x.(3x +7) = 0
⇔ \(\left[ \begin{array}{l}x=0\\3x+7=0\end{array} \right.\) ⇔ \(\left[ \begin{array}{l}x=0\\x=\frac{-7}{3}\end{array} \right.\)
Vậy S = { 0; $\frac{-7}{3}$ }