Okie, so you have your choice between simplest form as a mixed fraction or improper fraction. So lets just distribute out all the numbers to improper fractions instead of mixed.
7 4/9 = (7x9)+4 / 9 = 67/9
2 1/9 = (2x9)+1 / 9 = 19/9
Now, we subtract the two since they have the same denominator.
67/9 - 19/9 = 48/9. Now simplify.
48/9 = 16/3 yards. This is simplest as an improper fraction. As a mixed fraction it would be 5 1/3 yards. I hope this helps :-)