The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.5058\\[1 em]x_2 &= -2.4239\\[1 em]x_3 &= 3.263 \end{aligned} $$Polynomial $ 3x^3-x^2-25x-12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.