Well, let the radius of the smaller circle be r, so the radius of the larger circle is 2r-1. We then have

, since we just divided out the pi. Now, I just eyed it out and found r=4, so our answer is 7. If you can't see that, you can expand to get

, so we know r must be 4.

Or, after getting

, you can apply difference of squares to get

and you can either make it a quadratic again, or in this case it's easy to examine factors and find r=4.