a/ $\dfrac{2}{3} + x = -45$ %
$\dfrac{2}{3} + x = \dfrac{-45}{100}$
$x = \dfrac{-45}{100} - \dfrac{2}{3}$
$x = \dfrac{-335}{300}$
b/ $\dfrac{2}{9} - \dfrac{7}{8} . x = \dfrac{1}{3}$
$\dfrac{7}{8} . x = \dfrac{2}{9} - \dfrac{1}{3}$
$\dfrac{7}{8} . x = \dfrac{-1}{9}$
$x = \dfrac{-1}{9} : \dfrac{7}{8}$
$x = \dfrac{-8}{63}$