The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.4472\\[1 em]x_2 &= -0.4472\\[1 em]x_3 &= -2.2361\\[1 em]x_4 &= 2.2361 \end{aligned} $$Polynomial $ 5x^4-26x^2+5 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.