$\left\{\begin{array}{}x+3y=4\\2x+3y=-1\end{array}\right.$
$⇔\left\{\begin{array}{}\hspace{-1cm}3y=4-x\\2x+4-x=-1\end{array}\right.$
$⇔\left\{\begin{array}{}3y=4-x\\\hspace{-0,1cm}x=-5\end{array}\right.$
$⇔\left\{\begin{array}{}x=-5\\\hspace{-0,2cm}y=3\end{array}\right.$
$⇒(x;y)=(-5;3)$