I don't have the question on me but in a wittig reaction, all you have to remember is that the Carbon (along with whatever is attached to that carbon, and without any change in configuraiton) attached to the phosphorous in ylide will replace the oxygen of the carbonyl group, forming a C=C double bond.
There reaction goes through a intermediate (or maybe just a transition step i dont remember) that has a square of 4 bonds with (P-C-C-O-P(back to start)) as its 4 sides.