The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.457\\[1 em]x_2 &= -0.7285+0.9176i\\[1 em]x_3 &= -0.7285-0.9176i \end{aligned} $$Polynomial $ 4x^3-3x-8 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.