$\left \{ {{2x+y=-7} \atop {5x-3y=-34}} \right.$
⇔$\left \{ {{y=-7-2x} \atop {5x-3(-7-2x)=-34}} \right.$
⇔$\left \{ {{y=-7-2x} \atop {5x+21+6x=-34}} \right.$
⇔$\left \{ {{y=-7-2x} \atop {11x+21=-34}} \right.$
⇔$\left \{ {{y=-7-2x} \atop {11x=-55}} \right.$
⇔$\left \{ {{y=-7-2.(-5)} \atop {x=-5}} \right.$
⇔$\left \{ {{y=3} \atop {x=-5}} \right.$
Vậy hệ phương trình có nghiệm ( x ; y ) = ( -5 ; 3 )
