$a$) $12x-51=13$
⇔ $12x =13+51$
⇔ $12x =64$
⇔ $x=64:12$
⇔ $x$ =$\frac{16}{3}$
Vậy $x$ =$\frac{16}{3}$
$b$) |$7x+3$|=$66$
⇒\(\left[ \begin{array}{l}7x+3=66\\7x+3=-66\end{array} \right.\)
⇔\(\left[ \begin{array}{l}7x=63\\x=-69\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=9\\x=\frac{-69}{7}\end{array} \right.\)
Vậy $x$ ∈ {$\frac{-69}{7}$;$9$}
$c$) Bạn coi lại đề nhé!