The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 4i\\[1 em]x_2 &= -4i \end{aligned} $$The solutions of $ x^2+16 = 0 $ are: $ x = 4 i ~ \text{and} ~ x = -4 i $.
You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this quadratic equation.