The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 7\\[1 em]x_2 &= -13 \end{aligned} $$The solutions of $ x^2+6x-91 = 0 $ are: $ x = -13 ~ \text{and} ~ x = 7$.
You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this quadratic equation.