Answer:
If a point (x, y) is reflected across a line, image of the point will be at the same distance from the line as the original point is.
In fact line of reflection works like a mirror.
In the figure attached,
Distance of point B from the line 'm' = 6 units
Therefore, distance of the image point B' from line 'm' = 6 units (on opposite side of the line of reflection)
