(1) we assume law of excluded middle, so sets either contain or do not contain x.
(2) we also assume standard ZFC axioms, so in particular, A is well-formed formula:
A = {S in P(X): a in S} and B= {S in P(X): a not in S}
And it's left as an exercise to figure out why A U B = P(X) and A, B are...