Giải:
$a.7x+21=0$
$<=>7x=-21$
$<=>x=\frac{-21}{7}$
$<=>x=-3$
Vậy $x=-3$
$b.\frac{4}{3}x-\frac{5}{6}=\frac{1}{2}$
$<=>\frac{4}{3}x=\frac{1}{2}+\frac{5}{6}$
$<=>\frac{4}{3}x=\frac{1.6+5.2}{2.6}$
$<=>\frac{4}{3}x=\frac{16}{12}$
$<=>\frac{4}{3}x=\frac{4}{3}$
$<=>x=\frac{4}{3}:\frac{4}{3}$
$<=>x=1$
Vậy $x=1$