While I don't want to give away the answer, will help you eliminate a few answers.
First off, we'll look at B. This option states that you'd subtract the y coordinate from the first point from the x coordinate fro the second point. Does it make sense to subtract x-y?
Next, we can look at A. This states subtracting the x coordinate from the first point from the x coordinate of the second point. Does x-x make sense?
And finally, I'll explain D. This states adding the x coordinate from the first point from the x coordinate of the second point. Adding x+x? I don't think that makes sense.
I'd suggest choosing A. However, it is up to you.