Ta có :
$\left\{\begin{array}{l}3x+4y=-47\\3x+2y=-31\end{array} \right.$
$⇔\left\{\begin{array}{l}2y=-16\\3x+2y=-31\end{array} \right.$
$⇔\left\{\begin{array}{l}y=-8\\3x+2.(-8) = -31\end{array} \right.$
$⇔\left\{\begin{array}{l}y=-8\\x=-5\end{array} \right.$