The equation of XY plane is given by Z = 0

Given point (α,β,γ).

If we draw perpendicular from the point in the XY plane,

The coordinates of the point will be (α,β,0)

Let its reflection be (x,y,z)

So,

⇒ x = α

⇒ y = β

⇒ z = -γ

The reflection is:

(α , β ,-γ )