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You are correct there is no pattern
K=8k+28
subtract 8k from both side
-7k = 28
divide both side by -7
k= -4
Diameter= 2 times the radius
2r=d
2r=10.2
R= 10.2/2
R=5.1
Therefore, the radius is 5.1 units which makes the answer choice C correct.
Hopefully, this helps!