Если пара чисел (a; b) - решение системы x-y=1, 2x+y=-7, то сумма a + b чему будет равна?

[latex] \left \{ {{x-y=1} \atop {2x+y=-7}} \right. [/latex] [latex] \left \{ {{x=1+y} \atop {2(1+y)+y=-7}} \right. [/latex] [latex] \left \{ {{x=1+y} \atop {2+2y+y=-7}} \right. [/latex] [latex] \left \{ {{x=1+y} \atop {3y=-9}} \right. [/latex] [latex] \left \{ {{x=1+y} \atop {y=-3}} \right. [/latex] [latex] \left \{ {{x=1-3} \atop {y=-3}} \right. [/latex] [latex] \left \{ {{x=-2} \atop {y=-3}} \right. [/latex] Числа (a;b) это (-2;-3) а+b=-2-3=-5