$a) \frac{x}{3} = \frac{2}{3} + \frac{-1}{7}$
$⇒ \frac{x}{3} = \frac{11}{21} $
$⇒ x = \frac{11}{21} × 3$
$⇒ x = \frac{11}{7}$
$Vậy$ $ x = \frac{11}{7}$
$b) \frac{13}{4} + \frac{9}{10} = \frac{83}{x}$
$⇒ \frac{83}{20} = \frac{83}{x}$
$⇒ x = 83 ÷ \frac{83}{20}$
$⇒ x = 20$
$Vậy$ $x = 20$
