I have just started learning set theory and I've been trying to learn how to do proofs, however I really can't figure out
I've been trying to answer a simple one:
Let $A, B$ and $X$ be sets. Prove that if $A ∪ B ⊆ X$ then $A ⊆ X$.
If anyone can point me in the right direction (e.g. a strategy to use when going about these types of proofs) or provide a worked example of a different proof.