The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.7071i\\[1 em]x_2 &= -0.7071i\\[1 em]x_3 &= 2.6458i\\[1 em]x_4 &= -2.6458i \end{aligned} $$Polynomial $ 2x^4+15x^2+7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.