The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 10.4976\\[1 em]x_2 &= -3.2488+8.1027i\\[1 em]x_3 &= -3.2488-8.1027i \end{aligned} $$Polynomial $ -x^3+4x^2-8x+800 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.