[tex] \left \{ {{x+3y=7} \atop {3x-2y=-12}} \right. \\\\ \left \{ {{\boxed{x=7-3y}} \atop {3x-2y=-12}} \right. \\\\\\ 3(7-3y)-2y=-12 \\\\ 21-9y-2y=-12 \\\\ -11y=-12-21 \\\\ -11y=-33 \\\\ \boxed{\boxed{y=\frac{-33}{-11}=3}} \\\\ x=7-3y \\\\ x=7-3*3 \\\\ x=7-9 \\\\ \boxed{\boxed{x=-2}}[/tex]
