The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.5771\\[1 em]x_2 &= 1.2668\\[1 em]x_3 &= -2.5647 \end{aligned} $$Polynomial $ 8x^3+15x^2-20x-15 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.