The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 2.6654\\[1 em]x_2 &= -0.0411+0.466i\\[1 em]x_3 &= -0.0411-0.466i \end{aligned} $$Polynomial $ 12x^3-31x^2-7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.