The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -1.8845\\[1 em]x_2 &= 0.9423+2.9434i\\[1 em]x_3 &= 0.9423-2.9434i \end{aligned} $$Polynomial $ x^3+6x+18 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.