See photo attached (NEED SOON)

Answer:
Step-by-step explanation:
The formed triangles are similar and the ratio of corresponding sides is equal:
The light source should be at 3 m height
Both triangles are congruent so
[tex]\\ \bull\sf\longmapsto \dfrac{x}{4}=\dfrac{9}{12}[/tex]
[tex]\\ \bull\sf\longmapsto 12x=36[/tex]
[tex]\\ \bull\sf\longmapsto x=3[/tex]
Light source should be at 3m