The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -2.2843\\[1 em]x_2 &= 1.6421+0.8975i\\[1 em]x_3 &= 1.6421-0.8975i \end{aligned} $$Polynomial $ x^3-x^2-4x+8 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.