The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 2.6108\\[1 em]x_2 &= -2.1284\\[1 em]x_3 &= 11.5175 \end{aligned} $$Polynomial $ x^3-12x^2+64 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.