The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -3.2921\\[1 em]x_2 &= 0.1461+1.7147i\\[1 em]x_3 &= 0.1461-1.7147i \end{aligned} $$Polynomial $ 4x^3+12x^2+8x+39 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.