[tex]\displaystyle\bf\\\left \{ {{2x + y = 3} \atop {x + y = 0 }} \right. \\ \displaystyle\bf\\\left \{ {{2x - x = 3} \atop {y = - x }} \right. \\ \\ 2x - x = 3 \\ x = 3 \\ \\ y= - x = - 3[/tex]

Ответ: ( 3 ; - 3 )