a) $\frac{5}{7}$ + $\frac{2}{3}$x = $\frac{3}{10}$
$\frac{2}{3}$x = $\frac{3}{10}$ - $\frac{5}{7}$
$\frac{2}{3}$x = $\frac{-29}{70}$
x = $\frac{-29}{70}$ : $\frac{2}{3}$
x = $\frac{-87}{140}$
b) $\frac{3}{4}$x - $\frac{1}{2}$ = $\frac{3}{7}$
$\frac{3}{4}$x = $\frac{3}{7}$ + $\frac{1}{2}$
$\frac{3}{4}$x = $\frac{13}{14}$
x = $\frac{13}{14}$ : $\frac{3}{4}$
x = $\frac{26}{21}$