The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.8601\\[1 em]x_2 &= 0.4301+0.7449i\\[1 em]x_3 &= 0.4301-0.7449i \end{aligned} $$Step 1:
Combine like terms:
$$ \color{blue}{9x^3} \color{blue}{-20x^3} -7 = \color{blue}{-11x^3} -7 $$Step 2:
Polynomial $ -11x^3-7 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.