The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.6702\\[1 em]x_2 &= -0.8351+2.0229i\\[1 em]x_3 &= -0.8351-2.0229i \end{aligned} $$Step 1:
Combine like terms:
$$ x^3 \color{blue}{-2x} + \color{blue}{4x} -8 = x^3+ \color{blue}{2x} -8 $$Step 2:
Polynomial $ x^3+2x-8 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.