Verified answer

Just to make sure, I think that is n^2 ("n squared") + 3n ("three n") within the square root. If this is the case then the answer is:

0

The reason for this is that as n approaches infinity the "3n" part of the square root becomes irrelevant because n^2 is so much larger. This leaves you with sqrt(n^2)-n. At this point you can see that this simply reduces to n-n, or 0.