3.15 use the law of reflection (the angle of incidence equals the angle of reflection)

The angle of incidence is the angle that light hits a mirror; it is taken between 90 degrees from the mirror and the incidence wave (the wave that hits the mirror.)

The angle of reflection is the angle that light leaves the mirror; it is taken between 90 degrees from the mirror and the angle of reflection.

The angle of incidence is always the same as the angle of reflection.