The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.4483\\[1 em]x_2 &= -2.2627\\[1 em]x_3 &= -0.7595+2.1637i\\[1 em]x_4 &= -0.7595-2.1637i \end{aligned} $$Polynomial $ 3x^4+10x^3+21x^2+24x-16 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.