a cubical room ABCD has mirrored planar wall CD. Each side of room is 3m. We place a camera at mid point of the wall AB. At what distances should the camera be focussed to photograph an object placed at A ?

where is A placed?
Try n solve it by making a diagram of a square ABCD.....A is the point where the object is placed and CD has the mirror
oh A is a oint sorry
point sorry


Look at the diagram the camera takes the photograph of the image of the object 
it is located at 6.187 m (calculated by pythagroes theorem)
for good image size of image must be same as size of object
so object must be at 2f(image of A)
so 2f=6.187 m
f=3.0935 m