The roots of polynomial $ p(a) $ are:
$$ \begin{aligned}a_1 &= 8.3246\\[1 em]a_2 &= -4.3246 \end{aligned} $$The solutions of $ a^2-4a-36 = 0 $ are: $ a = 2-2 \sqrt{ 10 } ~ \text{and} ~ a = 2+2 \sqrt{ 10 }$.
You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this quadratic equation.