The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.9151\\[1 em]x_2 &= 3.1006\\[1 em]x_3 &= 0.4072+0.9442i\\[1 em]x_4 &= 0.4072-0.9442i \end{aligned} $$Polynomial $ -x^4+3x^3+3 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.