The venn diagram one is valid. Just draw 2 different pictures of 3 intersecting circles and label them "A" "B" and "C". One picture will be the left part of the equation and one will be the right. If they end up being the same, then it's true.
Then start shading in the circles one step at a time. A - B is everything in the A circle that isn't also in B (looks like a circle with a bite taken out of it). A - C' is kind of two steps. First C' is everything outside of the circle C. A - C' is the same as A intersection C, in other words the region where A and C overlap.
So keep going through and shading in areas and you'll end up with the same region on both sides, the circle A with a small chunk taken out of it.