The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -1.1865\\[1 em]x_2 &= 7.2814\\[1 em]x_3 &= 0.2859+0.2694i\\[1 em]x_4 &= 0.2859-0.2694i \end{aligned} $$Polynomial $ 3x^4-20x^3-15x^2+12x-4 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.