Here's a clear explanation:
First, remember that there's always an invisible 2 before the √ in a square root. You just don't see it because it makes no difference. If we wanted the cube root, there would be a 3 before the √.
So, basically, the 'n' in this case is a 2. 8 / 2 = 4.
I hope you understand me, or else, message me for further explanation.
No problem.
:D