She needs the difference between 3/4 and 1/3, i.e. (3/4) - (1/3)
The common denominator is 12 (and if you do not know how to get that, please write a comment which I will get and be able to help you further)
3/4 = 9/12 and 1/3 = 4/12; the difference you need then is (9/12) - (4/12) = 5/12
(Also if you do not know how to use the common denominator to make the equivalent fraction, please write a comment so that I can help you further).