From the mirror, It will appear as if the object has moved towards the glass block by t(1−1μ)=5(1−132)=53cm
So the apparent distance from the mirror will be 10+5−53=403cm
So image will form 403cm behind the mirror.
While looking at the image through the glass block from the opposite side of the mirror, it will again shift towards the mirror by 53cm
So the apparent distance will be 403−53=353cm
So, the distance of the final image with respect to the object is 15+353=803cm