$\left\{\begin{array}{1}-x-y=2\\-2x-3y=9\end{array}\right.$
$⇔\left\{\begin{array}{1}-2x-2y=4\\-2x-3y=9\end{array}\right.$
$⇔\left\{\begin{array}{1}y=-5\\-2x-3.(-5)=9\end{array}\right.$
$⇔\left\{\begin{array}{1}y=-5\\x=3\end{array}\right.$
Vậy phương trình có nghiệm duy nhất `(x;y)=(3;-5)`.