The points (9, -1) and (6, 1) could form a right triangle with (9,1) and (6, -1). If you plot the points on graph paper you will see that from (6, -1) to (6, 1)is a vertical line and from (6, 1) to (9, 1) Is a horizontal line. These form right angles. The same is true for the other pair of points.