The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 13.0412\\[1 em]x_2 &= -0.0206+0.7324i\\[1 em]x_3 &= -0.0206-0.7324i \end{aligned} $$Polynomial $ x^3-13x^2-7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.