The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 2.6494\\[1 em]x_2 &= -1.3247+1.1246i\\[1 em]x_3 &= -1.3247-1.1246i \end{aligned} $$Polynomial $ x^3-4x-8 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.