The first thing you need to know is that . One way of seeing this is to notice that choosing r objects out of n amounts to the same thing as (not) choosing the remaining n–r objects. Algebraically, both numbers are given by the same formula .

So , and your equation becomes

.

Cancel the factor n+1 that occurs in each term, multiply out what's left, and you'll get a simple equation for n.

The answer isSpoiler: