The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.569\\[1 em]x_2 &= 1.7845+1.9612i\\[1 em]x_3 &= 1.7845-1.9612i \end{aligned} $$Polynomial $ 4x^3-12x^2+20x+16 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.