Discrete Math Complement Proof

Question

I have never been good with proofs and its especially harder when the math is discrete. I have no idea on how to do this proof.

Show that if S1 ⊆ S2, then S¯2⊆S¯1 (the complement of S2 is the subset of the complement of S1)

Answer 1

Proof by contradiction. Let $x$ be a point in $S'_2$. If it is not in $S'_1$, then it must be in $S_1$ and therefore in $S_2$. But $x$ cannot be in both $S_2$ and $S'_2$.

Note: I am using ' to denote complement.