The triangle above has been rotated 90 degrees around the origin or point D. When on a coordinate plane you can follow the rules for rotations that effect the X and Y values. Without using a coordinate plane find the perpendicular bisector of a pair of corresponding points between the pre-image and the image. Where these bisectors intersect is where the center of rotation is. The angle from one corresponding point to another is the angle of rotation which should equal 90 degrees. This is how you can assure your rotation is correct.