The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.7737i\\[1 em]x_2 &= -1.7737i\\[1 em]x_3 &= 3.1391i\\[1 em]x_4 &= -3.1391i \end{aligned} $$Polynomial $ x^4+13x^2+31 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.