Actually, you can't. You could multiply the numerator and denominator of each 4/3 and 3/3 by 5, obtaining:
5(4) 5(3)
------ and --------
5(3) 5(3)
but this result has little practical value. 4/3 and 3/3 could be combined immediately, obtaining 7/3, if that's what you want to do, as the fractions already have the same denominator (3).
