The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.7655+1.2137i\\[1 em]x_2 &= 0.7655-1.2137i\\[1 em]x_3 &= -0.7655+2.5882i\\[1 em]x_4 &= -0.7655-2.5882i \end{aligned} $$Polynomial $ x^4+7x^2-8x+15 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.