The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -3.5386\\[1 em]x_2 &= 1.7693+1.1795i\\[1 em]x_3 &= 1.7693-1.1795i \end{aligned} $$Polynomial $ x^3-8x+16 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.