The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -3.9445\\[1 em]x_2 &= 4.4723+0.5296i\\[1 em]x_3 &= 4.4723-0.5296i \end{aligned} $$Polynomial $ x^3-5x^2-15x+80 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.