Read my question below.?

Suppose the three coordinate planes are all mirrored and a light ray given by the vector a=(a1,a2,a3) first strikes the xz-plane. Use the fact that the angle of incidence equals the angle of reflection to show that the direction of the reflected ray is given by b=(a1,-a2,a3). Deduce that, after being reflected by all three mutually perpendicular mirrors, the resulting ray is parallel to the initial ray.