Tìm x, y biết xy + 3x - 7y = 21

* Hướng dẫn giải

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