The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 2.2361i\\[1 em]x_2 &= -2.2361i\\[1 em]x_3 &= 4i\\[1 em]x_4 &= -4i \end{aligned} $$Polynomial $ x^4+21x^2+80 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.