The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -1.4614\\[1 em]x_2 &= 0.3557+0.5742i\\[1 em]x_3 &= 0.3557-0.5742i \end{aligned} $$Polynomial $ 12x^3+9x^2-7x+8 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.