The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 8.3222\\[1 em]x_2 &= -1.1611+1.7256i\\[1 em]x_3 &= -1.1611-1.7256i \end{aligned} $$Polynomial $ x^3-6x^2-15x-36 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.