Hello everyone!

I'm after this linear algebra question:

$\displaystyle A \text{ and }B \text{are subspaces of a vector space }V. \text{ show that }A\cup B \text{ are a subspace of }V \Leftrightarrow A\subseteq B \text{ or } B \subseteq A.$

Proving that $\displaystyle 0\in V $ is easy, howeve I'm not sure how to write down the proof for the other 2 axioms on paper though my professor cleared the entire matter up.

Thanks!