Moving an object and screen to find the focal length of a lens

[nofootcanman]

Homework Statement

A convex lens is used to project the image of an object on a screen with a magnification m₁ . Leaving the lens fixed, the object is moved along the optical axis
by a distance Δs and the screen is moved until the image is located again. If the new
value of the magnification is m₂ , Show that the focal lenghth \frac{Δs}{\frac{1}{m_{2}}-\frac{1}{m_{2}}}

Homework Equations

s'/s=m
1/s+1/s'=1/f
Where s is the position of the object and s' is that of the image/screen

The Attempt at a Solution

I used s₁=s₂+Δs and attempted to rearrange but got nowhere really.. any suggestions?

 

[mfb]

I used s₁=s₂+Δs and attempted to rearrange but got nowhere really.. any suggestions?

Please show your work then, so we can see what went wrong.