You can put this solution on YOUR website! The common factor is a number or term, in this case, that evenly divides both terms in your expression.
If I am going to divided 15x^4y and 5x^2y^3 by a factor that will produce a number or term with no remainder, what factor can that be?
It would have to 5x^2y. This term factors evenly into 15x^4y and 5x^2y^3.
If you don't believe me, try it out.