When you're trying to find something in the simplest form, you have to find a common denominator. The best way to do this is to find the factors.
Factors of 3= 1, 3
Factors of 28= 1,2,4,7,14,28
In this case, the lowest factor is one, which is what they are currently in.
Therefore, 3/28 is currently in its simplest form
