b
$\left|\begin{array}{ccc}{2 \alpha} & {1} & {5} \\ {1} & {-6} & {\alpha} \\ {1} & {1} & {2}\end{array}\right|=0$
$2 \alpha(-12-\alpha)-(2-\alpha)+5(7)=0$
$2 \alpha^{2}-24 \alpha-2+\alpha+35=0$
$2 \alpha^{2}-23 \alpha+33=0$
$\alpha_{1}+\alpha_{2}=\frac{23}{2}$