An easy way to do questions like these is by converting the fractions into decimals, at least I used to find it easier by doing so when I first encountered them.
So,
7/9 as a decimal is 0.77
1/3 as a decimal is 0.33
You can do;
0.77 - 0.33
You are left with;
0.44
Which in it's simplest fraction form is;
11/25.
Your answer is 11/25.
I hope this helped, if you need help with understanding how to conver then do let me know! ;3