The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 4.82\\[1 em]x_2 &= 8.09+1.2507i\\[1 em]x_3 &= 8.09-1.2507i \end{aligned} $$Polynomial $ x^3-21x^2+145x-323 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.