[tex]\left \{ {{y=-2x} \atop {y=4-x}} \right.\\-2x=4-x\\-2x+x=4\\-x=4\\x=-4\\y= -2*(-4)=8[/tex]

[tex]\left \{ {{3x+y=7} \atop {x+y=3}} \right. \left \{ {{3x+y=7} \atop {y=3-x}} \right. \left \{ {{3x-3-x=7} \atop {y=3-x}} \right. \left \{ {{2x=4} \atop {y=3-x}} \right. \left \{ {{x=2} \atop {y=3-x}} \right. \left \{ {{x=2} \atop {y=3-2}} \right. \left \{ {{x=2} \atop {y=1}} \right.[/tex]