Question

a5+a8=-43 ; a3=-11; d=?

Answer 1

$\displaystyle\bf\\\left \{ {{a_{5}+a_{8} =-43 } \atop {a_{3} =-11}} \right. \\\\\\\left \{ {{a_{1} +4d+a_{1} +7d=-43} \atop {a_{1} +2d=-11} \ |\cdot 2} \right. \\\\\\\left \{ {{2a_{1} +11d=-43} \atop {2a_{1}+4d=-22 }} \right. \\\\\\\left \{ {{-22+7d=-43} \atop {2a_{1} +4d=-22}} \right. \\\\\\7d=-21\\\\d=-3$