You need to find a common denominator. The lowest number that both 2 and 9 go into evenly. Since we know that 2 doesn't go evenly into 9 we need to try 18 because that is the next number that 9 goes into evenly. It happens to work because 2 will also go into it evenly.
Whatever we do to the top of the fraction we need to also do to the bottom. We can rewrite to problem like this
7x9 3x2
------ - -------
2x9 9x2
simplify this
63/18 - 6/18
Now we subtract the top numbers and get
57/18
you can either leave it in improper fraction form or you can convert it to mixed form.
for mixed number 18 goes into 57 3 times with a remainder of 3 so the mixed number would be
3 3/18 which can be reduced to 3 1/6