The answer to your question is 3x.
15x^6/5x^6
Divide 15 by 5 and since it's division you have to Subtract the exponent in denominator from the numerator.

BettieZumbrunnen

15 x^6 / 5 x^6
Divide numerator and denominator by x^6 : 15 / 5
Divide numerator and denominator by 5 : 3