Prove this set equality: $(S_1 \cup S_2) - S_1 = S_2 - S_1$ I am having trouble figuring out this problem. I have looked online for some identities and I have found one identity that has gotten me on my first step, but it hasn't helped me that much at all.
$(S_1 \cup S_2) - S_1 = S_2 - S_1$ (Original) 
$(S_1 \cup S_2) - S_1 = S_2 \cap S_1 '$
I don't know where to go from here. I have looked online for other identities and still don't know where to go from here.

$(S_1 \cup S_2)- S_1=(S_1 \cup S_2)\cap S_1'=(S_1 \cap S_1') \cup (S_2\cap S_1')=\phi \cup (S_2 -S_1)=S_2-S_1$
