The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 4.1039\\[1 em]x_2 &= -0.052+0.6509i\\[1 em]x_3 &= -0.052-0.6509i \end{aligned} $$Polynomial $ 4x^3-16x^2-7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.