$\displaystyle \text{Show } Z - (Y - X) = X \cup (Z - Y).$

I've tried this to no avail. The farthest I've gotten is $\displaystyle Z - (Y - X) = Z \cap (Y' \cup X)$, and I'm not sure how in the world to get that to equal the right side. Is there a mistake somewhere in the question? It looks like there's a Z missing on the right side of the original statement. (Yes, I looked at the book about ten times to make sure that I copied it right.)