If b is any other value, then the limit is no longer bounded. Do you see why?
I have another recommendation for getting the best help here, but only for your next question, no need to do this for this question:
Create a new topic for a new question. It is best to only post follow-up questions to the original problem in the same topic. I only mention these recommendations so that you will get the most prompt help.
We are given (assuming the limit exists):
Now, take the natural log of both sides, and use , and using the properties of logs, see if you can get one of the forms for which L'Hôpital's rule may be applied.