Another tip for getting started on problems like these: if you don't know where to start just plug some stuff in. In this case you want to solve for n but what happens if you specify n ahead of time? If n = 1 can you figure out what the probability on the left hand side is? You have everything you need to be able to calculate it. What about for n = 2? Once you do a few examples maybe that will help you solve the general case for leaving n unknown and solving for it.