This is a trival question. Are you sure you have stated it correctly.
I apologize, I don't know how to type these symbols in math code.
The way I think of this is "The elements are either in A or in B, and they are not in B." So I would think that all the elements must be in A, but apparently this is not the case if it is asking me to find two sets where this is not true.Give an example of two sets A and B for which (A U B) \ B /= A