- image distance=-40cm=v
- Object distance=u=?
- Magnification=m=3
We know
[tex]\boxed{\sf m=-\dfrac{v}{u}}[/tex]
[tex]\\ \sf\longmapsto 3=-\dfrac{-40}{u}[/tex]
[tex]\\ \sf\longmapsto 3=\dfrac{40}{u}[/tex]
[tex]\\ \sf\longmapsto u=\dfrac{40}{3}[/tex]
[tex]\\ \sf\longmapsto u=13.3cm[/tex]
