Hi/Ho =9/3=3X
Then: Di/Do = 3 X → Di = 3 Do
Let distance between mirror and wall = Z. Then Z = Di (because image is on wall).
Then Z=3Do
But Do +3=Z→ Do + 3=3Do → 3=2Do → Do=1.5m and Di=4.5m
Therefore, mirror must be placed at 4.5 m from wall.
Using mirror equation, we can calculate focal length: 1/f=1/1.5+1/4.5 →
f=4.5/4=1.125m