The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -2.5568\\[1 em]x_2 &= -0.2216+2.412i\\[1 em]x_3 &= -0.2216-2.412i \end{aligned} $$Polynomial $ x^3+3x^2+7x+15 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.