In order to add or subtract mixed numbers, you first need to
change them to improper fractions.
5-2/3 = 17/3
2-3/4 = 11/4 .
Now, in order to add or subtract fractions, you need to write
them over a common denominator.
17/3 = 68/12
11/4 = 33/12
Now you're ready to subtract:
68/12 - 33/12 = 35/12 = 2-11/12