The equality x-x=0 and 0=x-x are suppose to be the same. The first equality is easy to understand while the second equality( 0=x-x )is somewhat mind boggling to me for the following reason: where do the 2x's on the right side come from?
Assuming I understand your question: They come from the same "place" in each equation, namely, from anywhere at all. It might help to think of it this way: "What's the result of subtracting any magnitude at all from itself? Zero." "What's zero? The result of subtracting any magnitude at all from itself." Each answer is just as good as the other in answering the respective question being asked.