Solution for a quadratic equation The following is the solution from my book, but without the calculation path.
$$
\frac{(18 \pm \sqrt{-180})}{6} = 3 \pm\sqrt{-5} 
$$
Why is it $-5$?

$$\begin{align}
\frac{1}{6}(18\pm\sqrt{-180}) 
&= \frac{1}{6} \times (18) \pm \frac{1}{6} \times \sqrt{-180}\\
&= \frac{1}{6} \times (18) \pm \frac{1}{6} \times \sqrt{-4 \times 9 \times 5}\\
&=\frac{1}{6} \times (18) \pm \frac{1}{6} \times (2 \times 3) \times\sqrt{-5}\\
&= 3 \pm\sqrt{-5}
\end{align}$$
