You have to make common denominators by multipying the 1/3a by 2b/2b 1/2b by 3a/3a and distribute the negative through the ( ) making -2b+3a.
This gives you (2b +3a - 2b -3a) /6ab
Which simplifies to 0/6ab which is 0.
If you get confused, drop numbers in for a and b. Pick primes that are not already in the problem -- just don't multiply it out then convert back to a and b when you are done. This is the same process as adding and subtracting numerical fractions.