$a) x×\dfrac{-5}{8} =\dfrac{15}{32}$ 

$⇔x=\dfrac{15}{32}÷\dfrac{-5}{8}$ 

$⇔x=\dfrac{15}{32}×\dfrac{-8}{5}$ 

$⇔x=\dfrac{3×(-1)}{4×1}$ 

$⇔x=\dfrac{-3}{4}$

Vậy $x=\dfrac{-3}{4}$