Since we know this figure is a square, we also know that all sides are the same. Therefore, we must figure out one side length and then use that math to find the other point. A simple way to do this is to graph the points and match them up for the missing corner.
In the graph below, you will see that (-6, -3) is circles in red, showing that it is the correct answer. The red dots show how we could match up the corners to find the answer.
