You should start by finding the distribution of the sample mean (and hence ).

I think this is done somewhere in MHF - you will have to search for it. Alternatively, I'm sure it will be somewhere on the web and certainly many textbooks do it. And you could always have a go at doing it yourself. If you're truly stuck with this, please say so (after you've made an effort to try and find it).

Then use the usual formulae for finding and .