The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.3905\\[1 em]x_2 &= -0.6398\\[1 em]x_3 &= 400.2492 \end{aligned} $$Polynomial $ x^3-400x^2-100x+100 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.