a, x × $\frac{7}{9}$ = $\frac{2}{3}$ + 2$\frac{1}{2}$
x × $\frac{7}{9}$ = $\frac{19}{10}$
x = $\frac{19}{10}$ : $\frac{7}{9}$
x = $\frac{171}{70}$
b, $\frac{5}{7}$ + x : $\frac{9}{4}$ = $\frac{4}{3}$
x : $\frac{9}{4}$ = $\frac{4}{3}$ - $\frac{5}{7}$
x : $\frac{9}{4}$ = $\frac{13}{21}$
x = $\frac{13}{21}$ × $\frac{9}{4}$
x = $\frac{39}{28}$