8 is the answer

6a^m/2a^3

The 6 divides by the 2 to give you the 3, then you use you exponent rules where the exponent from the a in the denominator subtracts from the exponent of the a in the numerator to give you the new exponent (^8-^3=^5) so the whole this will give you 3a^5

Sorry if that didn't make sense, I'm not great at explaining math. If you didn't understand im sure there's someone here who will explain it better