$\left \{ {{x+2y=4} \atop {2x+7y=17}} \right.$
⇔$\left \{ {{2x+4y=8} \atop {2x+7y=17}} \right.$
⇔$\left \{ {{-3y=-9} \atop {2x+7y=17}} \right.$
⇔$\left \{ {{y=3} \atop {2x+7y=17}} \right.$
⇔$\left \{ {{y=3} \atop {2x+7.3=17}} \right.$
⇔$\left \{ {{y=3} \atop {2x+21=17}} \right.$
⇔$\left \{ {{y=3} \atop {2x=-4}} \right.$
⇔$\left \{ {{y=3} \atop {x=-2}} \right.$