$a$) $\dfrac{3}{2}.x - \dfrac{1}{2}.x = \dfrac{5}{12}$
$⇔ (\dfrac{3}{2} - \dfrac{1}{2}) .x = \dfrac{5}{12}$
$⇔ \dfrac{2}{2} . x = \dfrac{5}{12}$
$⇔ x = \dfrac{5}{12}$
Vậy $x = \dfrac{5}{12}$
$b$) $(2\dfrac{4}{5}x - 50) : \dfrac{2}{3} = 51$
$⇔ 2\dfrac{4}{5}x - 50 = 34$
$⇔ 2\dfrac{4}{5}x = 84$
$⇔ \dfrac{4}{5}.x = 42$
$⇔ x = \dfrac{105}{2}$
Vậy $x = \dfrac{105}{2}$
