I made a Venn Diagram so I know that this is true. Now I just need some help on getting the proof right.
What I did first was obviously assume the premises, and then I tried to unpack them. So now I have $x\in A \to x\in B$ because $A \subseteq B$ and $x\in A \to x\in C$ because $A \subseteq C$.
Any pointers on where I should go from here?