The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.1723\\[1 em]x_2 &= -3.0862+0.8436i\\[1 em]x_3 &= -3.0862-0.8436i \end{aligned} $$Polynomial $ -x^3-5x^2-3x+12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.