(3^2n x 25^2n-1) / (15^n-1)
= (3^2n x 5^(4n-2) / ((3x5)^n-1)
= (3^2n x 5^(4n-2) / ((3^n-1) x (5^n-1))
= (3^n+1) x (5^3n-1)
If the working above is not right, you need to apply your index laws to simplify the numbers down to 3s and 5s, before cancelling. I hope I'm right...