Please show me what you tried.
If you don't provide the information requested I cannot help you.
The way to get the most out of this forum is to, first, show us your working as best as you can manage. Then we can help you over the stuck part. The idea is that you will learn physics best by doing the problem yourself.
Also try to be patient when waiting for replies - in general we will not be waiting by the computer for you to get back and answer questions. It is likely that you will not get a reply for 10-20 hours. Thus, the more you can tell us about your problem, at the start, and how you have been thinking about it the sooner you can get a useful reply. (You should factor this into your thinking when you post a question too.)