The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.5963\\[1 em]x_2 &= -4.2981+1.2852i\\[1 em]x_3 &= -4.2981-1.2852i \end{aligned} $$Polynomial $ x^3+8x^2+15x-12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.