Proposition 3.15?

Draw a picture of two perpendicular lines. Lable two of the adjacent angles 1 and 2. Also show that one of the angles is a right angle. Since angle 1 and 2 are adjacent, they are supplementary. That means they = 180 degrees. If one of the angles is a right angle it equals 90 degrees, therefor by the subtraction property the other angle must equal 90 degrees and would also be a right angle by definition of a right angle.