The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -8.6408\\[1 em]x_2 &= 4.3204+11.2249i\\[1 em]x_3 &= 4.3204-11.2249i \end{aligned} $$Polynomial $ 4x^3+280x+5000 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.