Prove:
A × B = ∅ if and only if A = ∅ or B = ∅.
i would do this by proving the forward direction using the contrapositive, and the converse via a direct proof.
at some point in your text you should have it written that $\displaystyle A \times \emptyset = \emptyset$ for every set $\displaystyle A$. this should help you prove the converse directly.
for the forward direction, assume NOT $\displaystyle A = \emptyset$ or $\displaystyle B = \emptyset$. then by DeMorgan's laws, we have that $\displaystyle A \ne \emptyset$ AND $\displaystyle B \ne \emptyset$. now if both $\displaystyle A$ and $\displaystyle B$ are nonempty, what can you say about $\displaystyle A \times B$?