By definition, A∖B is all those elements in A which are not in B. We can read them: 3,6,9,15,18,21. Hence
A∖B={3,6,9,15,18,21}.
Similarly,
B∖A={4,8,16,20}.
What is A∪(B∖A)? Observe
{3,6,9,12,15,18,21,24}∪{4,8,16,20}={3,4,6,8,9,12,15,16,18,20,21,24}.
This is precisely A∪B. Do you also observe A∩(B∖A)=/0?