There is a certain method to go about doing this. I remember the start and finish, but not the intermediate steps.
Let: X ~ Poisson( )
Or something like it. It had to do with the fact that E[X] in this case =
... ... ... ...
Try reading this thread: http://www.mathhelpforum.com/math-he...-one-more.html
Then let me know if you're still stuck.