The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.6317\\[1 em]x_2 &= 2.2411\\[1 em]x_3 &= 14.1272 \end{aligned} $$Polynomial $ x^3-17x^2+42x-20 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.