The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 2.3836\\[1 em]x_2 &= -2.7623\\[1 em]x_3 &= 6.3787 \end{aligned} $$Polynomial $ -x^3+6x^2+9x-42 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.