Roots of Resolvent of Cubic

Theorem
Let $P$ be the cubic equation:
 * $a x^3 + b x^2 + c x + d = 0$ with $a \ne 0$

Let $r \left({P}\right)$ be the resolvent equation of $P$, given by:
 * $u^6 - 2 R u^3 - Q^3$

Let the roots of $P$ be $\alpha_1, \alpha_2, \alpha_3$.

Then the roots of $r \left({P}\right)$ can be expressed as: