The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.34\\[1 em]x_2 &= 0.34\\[1 em]x_3 &= 0.8611\\[1 em]x_4 &= -0.8611 \end{aligned} $$Polynomial $ 35x^4-30x^2+3 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.