$a) (2x -1)(3x +6) = 0$
$⇔ \left[ \begin{array}{l}2x -1=0\\3x +6=0\end{array} \right. ⇔ \left[ \begin{array}{l}x=\frac{1}{2}\\x=-2\end{array} \right.$
$Vậy$ $S =$ {$\frac{1}{2}; -2$}
$b) \frac{3x -2}{4} = \frac{2x -3}{3}$
$⇔ \frac{3.(3x -2)}{12} = \frac{4.(2x -3)}{12}$
$⇔ 3.(3x -2) = 4.(2x -3)$
$⇔ 9x -6 = 8x -12$
$⇔ 9x -8x = -12 +6$
$⇔ x = -6$
$Vậy$ $S =$ {$-6$}