The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.0607+1.0607i\\[1 em]x_2 &= 1.0607-1.0607i\\[1 em]x_3 &= -1.0607+1.0607i\\[1 em]x_4 &= -1.0607-1.0607i \end{aligned} $$Polynomial $ 16x^4+81 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.