The best way to find the simplest form a fraction is to find a common denominator- to do this, I would recommend listing all of the multiples of the two denominators.
Multiples of 9- 9 18 27 36 45 54
Multiples of 15- 15 30 45 60
The common denominator can be 45.
For the first fraction, you need to multiply everything by 5 (as 45/9=5), which gives you 40x5=20/ 9x5=45= 20/45.
The second fraction, you need to multiply everything by 3 (45/15=3), which gives you 2x3=6/ 15x3=45 =6/45
As you're subtracting them, you just need to subtract the numerators, 20-6=14
14/45 cannot be simplified any further, therefore, this is the simplest form
Hope this helps :)