The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.0596\\[1 em]x_2 &= -0.4791\\[1 em]x_3 &= -0.1088+0.6366i\\[1 em]x_4 &= -0.1088-0.6366i\\[1 em]x_5 &= 1.3782+0.314i\\[1 em]x_6 &= 1.3782-0.314i \end{aligned} $$Polynomial $ 42x^6-84x^5+20x^4+8x^3+21x^2+18x+1 $ has no rational roots that can be found using Rational Root Test, so the roots were found using Newton method.