This is wrong. What you have written is 2 times
m while, in fact, A has 2m
subsets. If you don't want to use LaTeX (as you have below) use 2^m.
That's correct but seems overly complicated. If A and B have no members in common, [itex]A\cup B[/itex] contains every member of A and
B- thus has m+ n members.