It's hard to read your image, unfortunately. Seems to me that it states that C=50.2 and that the formula for circumference is C=2pi*r.
50.2
In that case, C= 50.2 = 2(3.14)r, and r = ------------- = 7.99.
6.28
We are to round this to one decimal place. That results in r = 8.0.