Tìm x, y biết xy + 3x - 7y = 21

xy+3x−7y=21⇔xy+3x−7y−21=0⇔x(y+3)−7(y+3)=0⇔(x−7).(y+3)=0⇒x−7=0y+3=0⇔x=7y=−3