Actually the circumference of a circle is :
C =2Pi*r
Where r = radius, the radius term r, was missing in your expression.
C = 2Pi*r
7.68 = 2*3.14*r
2*3.14*r = 7.68
6.28r = 7.68
r = 7.68 / 6.28
r ≈ 1.223
So in solving for the missing variable, r ≈ 1.223
Hope this explains it.