The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.4891\\[1 em]x_2 &= 4.224\\[1 em]x_3 &= -8.7131 \end{aligned} $$Polynomial $ x^3+4x^2-39x+18 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.