When they \) questions like this, they are \)ing the distance from the point to the closest point on the plane, since that is the shortest distance. If you drew a line between these two points, it would be perpendicular (""normal"") to the plane.
The distance of a point (x, y, z) to the xy-plane is the value of the z-coordinate. Where negative values would indicate that the point was on the negative z side of the xy-plane. If you needed to only know the distance though you could take the absolute value.
In the case of (4,-2,6), it is 6 units from the xy-plane because z=6.
(closest point on the plane is (4,-2,0))
Result:
6