The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -10.4931\\[1 em]x_2 &= 0.7465+0.3313i\\[1 em]x_3 &= 0.7465-0.3313i \end{aligned} $$Polynomial $ x^3+9x^2-15x+7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.