a) $\left \{ {{2x+y=7} \atop {x-3y=-7}} \right.$ ⇔ $\left \{ {{2x+y=7} \atop {2x-6y=-14}} \right.$
⇔ $\left \{ {{2x+y=7} \atop {-7y=-21}} \right.$ ⇔ $\left \{ {{2x+y=7} \atop {y=3}} \right.$
⇔ $\left \{ {{x=2} \atop {y=3}} \right.$
b) $\left \{ {{2x-y=1-2y} \atop {3x+y=3-x}} \right.$ ⇔ $\left \{ {{2x+y=1} \atop {4x+y=3}} \right.$
⇔ $\left \{ {{4x+2y=2} \atop {4x+y=3}} \right.$ ⇔ $\left \{ {{y=-1} \atop {4x+y=3}} \right.$
⇔ $\left \{ {{y=-1} \atop {x=1}} \right.$
