1)Expand the bottom out.
2)Divide both the top and bottom of the fraction by
And you should arrive at:
You know that right? So anything like you can express as or even
So the limit in your question is
There are a variety of ways. I think the clearest at this stage of the game would be to expand the denominator.
The next step is standard: Divide the numerator and denominator by the highest power appearing in the problem, in this case :
Now take your limit.