Hey guys. New to the math forum. I'm having trouble proving this question. If someone can give just a guide on how to start it, that would assist me with this question and 4 others hopefully.

Let A, B, C be sets. Show that (A∪ B) ⊆ (A ∪ B ∪ C)

I started to to prove the right side by switching the parenthesis, using Associative Law. I used x e A or x e B or x e C (or = the logic upside down carrot symbol)

What do I do from their to prove the left side? Thanks for any input.