The third and thus last solution of $x^3 + \alpha x + \beta = 0$ is
\[x = \frac{(1-i \sqrt{3}) \alpha}{\sqrt[3]{12} \cdot t}
     -\frac{(1+i\sqrt{3}) t}{2\sqrt[3]{18}}\]

The complex conjugate root theorem states
that if $x$ is a complex root of a polynomial $P$, then its
complex conjugate $\overline{x}$ is also a root of $P$.
The solution presented in this case is the complex conjugate of
case 2.2.