Решить система уравнений

[latex] \left \{ {{(x-y)(x+y)=91} \atop {y+x=-7}} \right. ; \left \{ {{-7(x-y)=91} \atop {y+x=-7}} \right. ;\left \{ {{x-y=-13} \atop {y+x=-7}} \right. ;\left \{ {{2x=-20} \atop {y+x=-7}} \right. ;x=-10;y=3[/latex]