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.
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.