The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -1.7065\\[1 em]x_2 &= 2.3533+1.2223i\\[1 em]x_3 &= 2.3533-1.2223i \end{aligned} $$Polynomial $ -x^3+3x^2+x-12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.