So this is actually pretty easy. If you look at (3,3) and (3,-3) it is a mirrored affect. So what I did when I was doing this, is I did the same thing to the other side. The opposite coordinates of (1, -3) are (-1,3). So your answer is (-1,3).
All you need to do is finish the rectangle. if you look, the (3,-3) when moved up becomes positive (3,3). knowing this, you can do the same for the missing vertex, making both the numbers positive, instead of (1,-3), make them both positive and make the last vertex on (1,3).
