The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 6.658\\[1 em]x_2 &= 1.171+2.0089i\\[1 em]x_3 &= 1.171-2.0089i \end{aligned} $$Polynomial $ x^3-9x^2+21x-36 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.