Because you need to have the same denominators to subtract fractions, you need to find the LCM of 15 and 9. The LCM of 15 and 9 is 45. Because 15 goes into 45 3 times, you multiply 2 3 to find the numerator. Because 9 goes into 45 5 times, you multiply 55 to find the numerator.
The new equation is 25/45 - 6/45. The answer is 19/45, which cannot be simplified any further.