If you're not sure, begin by looking for any divisor that will divide into 24a^3c and 3a without leaving a remainder. Note that 3 is such a number, and a is another.
Factoring out 3a from 24a^3*c and 3a, we get 3a{8a^2*c, 1}
So the GCF is 3a.
The answer is would have to be b.
I believe it would be 6 due to the radius being a half of the actual circumfrence
