The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.0267\\[1 em]x_2 &= -0.9031\\[1 em]x_3 &= 1.4382+4.4162i\\[1 em]x_4 &= 1.4382-4.4162i \end{aligned} $$Polynomial $ x^4-3x^3+21x^2-20 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.