Find the distance of final image from mirror.

Explanation

(a) Shift by glass slab `=6(1-(1)/(3//2))=2cm`
The mirror will be shifted towards slab by `2cm`

Distance of object from virtual mirror `=18cm`
Image of O is formed at distance `18cm` behind the virtual mirror `M^'M_0^'`.
Distance of image from real mirror `MM_0=18-2=16cm`.
(b)
Same answer as in (a).
The distance of image from mirror is independent of separation between slab and mirror.