The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.9579\\[1 em]x_2 &= -2.6333\\[1 em]x_3 &= 1.1289+0.558i\\[1 em]x_4 &= 1.1289-0.558i \end{aligned} $$Polynomial $ 3x^4+4x^3-12x^2+12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.