The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.4026\\[1 em]x_2 &= -0.2013+0.1469i\\[1 em]x_3 &= -0.2013-0.1469i \end{aligned} $$Polynomial $ -40x^3+4x+1 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.