The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -1.5484\\[1 em]x_2 &= 1.0242+1.1717i\\[1 em]x_3 &= 1.0242-1.1717i \end{aligned} $$Polynomial $ 4x^3-2x^2-3x+15 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.