The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 2.4495i\\[1 em]x_2 &= -2.4495i\\[1 em]x_3 &= 1.7321i\\[1 em]x_4 &= -1.7321i \end{aligned} $$Polynomial $ x^4+9x^2+18 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.