Find the distance between the point (-3,4,-6)and its image in the XY plane. Pls answer fast..dont give any links..pls give explanation

If a point of an object has coordinates (x, y, z) then the image of this point (as reflected by a mirror in the xy plane) has coordinates (x, y, -z).
Hence the coordinate of the image of the point(-3,4,-6) in xy-plane will be (-3,4,6).
So the distance between them will be d = $$\begin{gathered} \sqrt{({\mathrm{x}}_2-{\mathrm{x}}_1{)}^2+({\mathrm{y}}_2-{\mathrm{y}}_1{)}^2+({\mathrm{z}}_2-{\mathrm{z}}_1{)}^2} \\ =\sqrt{{\left(-3+3\right)}^2+{\left(4-4\right)}^2+{\left(6+6\right)}^2} \\ =\sqrt{(6+6{)}^2} \\ =12 \end{gathered}$$