a)
$3x$ - $15$ - $6$
⇒ $3x$ - $21$
⇒$3$ ( $x$ - $7$ )
b)
$7$ - $x$ = - $2x$ + $3$
⇒ - $x$ + $2x$ = $3$ - $7$
⇒ $x$ = - $4$
$S$ = { - $4$ }
c)
($2x$ + $4$) ($5x$ + $15$)=$0$
⇒\(\left[ \begin{array}{l}2x+4=0\\5x+15=0\end{array} \right.\)
⇒\(\left[ \begin{array}{l}x=-2\\x=-3\end{array} \right.\)
Vậy $S$ = { - $2$ : - $3$ }