The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.6565\\[1 em]x_2 &= -0.6565\\[1 em]x_3 &= -2.3599\\[1 em]x_4 &= 2.3599 \end{aligned} $$Polynomial $ 5x^4-30x^2+12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.