The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 3.5905\\[1 em]x_2 &= -0.0453+0.5682i\\[1 em]x_3 &= -0.0453-0.5682i \end{aligned} $$Polynomial $ 6x^3-21x^2-7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.