The roots of polynomial $ p(d) $ are:
$$ \begin{aligned}d_1 &= 5.0721\\[1 em]d_2 &= -0.0361+1.5377i\\[1 em]d_3 &= -0.0361-1.5377i \end{aligned} $$Polynomial $ d^3-5d^2+2d-12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.