You have to make the denominator common before subtracting 2 fractions.
Since the end fraction has a denominator of 6 try and multiply both of the ones on the LHS to get 6 as a denominator.
First though convert the improper fractions into fractions solely.
7 1/2 goes to 15/2
While
4 2/3 goes to 14/3
Now
15/2 multiply both numerator and denominator by 3 gives you 45/6
14/3 multiply both numerator and denominator by 2 gives you 28/6
Subtract 45/6 by 28/6= 17/6
Highest multiple of 6 in 17 is 12
12/6 + 5/6 = 2 5/6
