The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.8284\\[1 em]x_2 &= -4.8284\\[1 em]x_3 &= 1.5+1.3229i\\[1 em]x_4 &= 1.5-1.3229i \end{aligned} $$Polynomial $ x^4+x^3-12x^2+28x-16 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.