Why does the negative sign leave when this expression is simplified?

Question

I was working on some math problems on Khan Academy and wondered why does the negative '-' sign go away when the following radical expression is simplified? In particular how does one obtain the equality $$\frac{-4}{-6} \pm \color{red}{-}\frac{2\sqrt{7}}{6} = \frac{2}{3} \mp \frac{7}{\sqrt{3}}$$

Answer 1

We have $$a\pm -b = \begin{cases}a+ -b = a-b\\ a--b = a+b\end{cases} = a\mp b;$$can you see how this applies to your situation?