The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.6713\\[1 em]x_2 &= 0.3356+1.0023i\\[1 em]x_3 &= 0.3356-1.0023i \end{aligned} $$Polynomial $ -12x^3-8x-9 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.