Answer
see the detailed answer
Work Step by Step
This is just a restatement of the definition.
In A-B there are the elements that are in set A but not in set B, in similar B-A have all the elements of B that are not in A,
so when union is applied on set A-B and set B-A the result have all the elements of set A and set B except those of common elements of both A and B.