The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -115.1915\\[1 em]x_2 &= -2.4043+60.3351i\\[1 em]x_3 &= -2.4043-60.3351i \end{aligned} $$Polynomial $ x^3+120x^2+4200x+420000 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.