The man should walk 7.5 m towards the mirror.
The reason being, the image formed by a plane mirror is at the same distance behind the mirror as the object is from the mirror. A distance of 5m between man and his image means that the distance between him and the mirror = 5/2= 2.5 m.
Thus, the distance he should walk = 10 - 2.5 = 7.5 m.