Помогите срочно номер 4

Решите задачу:

$\left \{ {{a_5+a_7=18} \atop {a_2=-3}} \right. \; \left \{ {{(a_1+4d)+(a_1+6d)=18} \atop {a_1+d=-3}} \right. \left \{ {{2a_1+10d=18} \atop {a_1=-3-d}} \right. \\\\ \left \{ {{2(-3-d)+10d=18} \atop {a_1=-3-d}} \right. \; \left \{ {{8d=24} \atop {a_1=-3-d}} \right. \; \left \{ {{d=3} \atop {a_1=-6}} \right. \\\\a_6=a_1+5d=-6+5\cdot 3=-6+15=9$