Now we have already given n(A)=3 and n(B)=6
So n(A∪B) can be minimum when n(A∩B)) is maximum or these two sets have maximum overlapping or in other words maximum elements in common. AA and BB can have atmost 3 elements common.
Then minimum no. of elements of A∪B can be achieved only when A⊂B . Note here it is proper subset symbol.
Now max value can be achieved when n(A∩B) is minimum or n(A∩B)=0