The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.7924\\[1 em]x_2 &= -0.6462+1.4514i\\[1 em]x_3 &= -0.6462-1.4514i \end{aligned} $$Polynomial $ 2x^3+x^2+3x-4 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.