What is the greatest common factor of and 3a

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.