The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -2.2886\\[1 em]x_2 &= -0.7307+0.3486i\\[1 em]x_3 &= -0.7307-0.3486i \end{aligned} $$Polynomial $ 4x^3+15x^2+16x+6 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.