\left\{\begin{array}{l} y=7-x \\ x+3 y=7 \end{array}\right. \left\{\begin{array}{l} 4 x+3 y=-16 \\ -x+y=4 \end{array}\right. \left\{\begin{array}{l} 2 x-4 y=-4 \\ 3 x-y=4 \end{array}\right.