${xy}$ + ${3x}$ - ${7y}$= ${21}$
⇒ ${( xy + 3x )}$ - ${7(y + 3 - 3)}$= ${21}$
⇒ ${x(y+3)}$ - ${7(y+3)}$ + ${21}$= ${21}$
⇒ ${x(y+3)}$ - ${7(y+3)}$ + ${21}$ = ${21}$ - ${21}$
⇒ ${x(y+3)}$ - ${7(y+3)}$ + ${21}$ = ${0}$
⇒ ${( x - 7)}$ . ${(y+3)}$ = ${0}$
⇒\(\left[ \begin{array}{l}x-7=0\\y+3=0\end{array} \right.\)
⇒ \(\left[ \begin{array}{l}x=7\\y=-3\end{array} \right.\)
$\text{Vậy}$ \(\left[ \begin{array}{l}x=7\\y=-3\end{array} \right.\)
