The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.6622\\[1 em]x_2 &= -0.6622\\[1 em]x_3 &= -2.1358\\[1 em]x_4 &= 2.1358 \end{aligned} $$Polynomial $ x^4-5x^2+2 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.