A candle flame 3cm is placed at distance of 3m from a wall. How far from wall must a concave mirror be placed in order that it may form an image of flame 9cm high on the wall

450 cm

Let the mirror be placed at a distance x from wall

By using

$\frac{1}{O}=\frac{-v}{u} \Rightarrow \frac{-9}{+3}=\frac{-(-x)}{-(x-3)}$ (image will be inverted)

⇒ x = −4.5 m = −450 cm.