# Need some help

suppose not. Then there would be an $x\in B \cap A$. But this means x is in A and x is in B. But since x is in B, and $B \subset A^c$, then x is in the complement of A. But then we have $x\in A^c$ and $x \in A$, which is clearly a contradiction as $A \cap A^c = \emptyset$