The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 5.3769\\[1 em]x_2 &= -5.1884+5.3736i\\[1 em]x_3 &= -5.1884-5.3736i \end{aligned} $$Polynomial $ x^3+5x^2-300 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.