The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 8.1065\\[1 em]x_2 &= -0.0533+0.9277i\\[1 em]x_3 &= -0.0533-0.9277i \end{aligned} $$Polynomial $ -x^3+8x^2+7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.