Given a*b*c prove that ab=ac completing the proof of prop 3.6 deduce that every ray has a unique opposite ray

The said triangle that is asked is a right triangle which has a proportion on each angle of 3 and 6. These proportions are the angles, which is 45, 45, and 90, which also means that the two rays of the triangle are the same and proved that ab = ac that makes the angle a is a 90 degree in value.