The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 5.8766\\[1 em]x_2 &= -2.4383+1.3085i\\[1 em]x_3 &= -2.4383-1.3085i \end{aligned} $$Polynomial $ x^3-x^2-21x-45 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.