In the case of a plane mirror, the distance between the object and the mirror (d1) is same as the distance between the image and the mirror (d2).

Distance between the mirror and David's image, d2 = 4 m

Therefore, d1 = d2 = 4 m

If David moves 1 m towards the mirror, then d1 = 4 − 1 = 3 m

Again, d1 = d2 = 3 m

Therefore, the distance between David and his image is d1 + d2 = 3 + 3 = 6 m.