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 Homework Statement:
 So in my physics assignment we had to graph the negative of magnification (M) versus distance between image and lens (q). From this we had to find the focal lengths from the intercepts. I have the focal length using the x intercept of the equation M=1q/f (which I rearragened to M= q/f 1) but I can’t figure out how to find the focal length using the y intercept.
 Relevant Equations:

Y=mx+b is equivalent to M=1/f•q 1
Focal point using x intercept: x0=b/m=f
I tried M=pq = f/(fp) = f/f  f/p = 1(f/p)
=> M+1= f/p
=> Mpp=f
But since it’s the y intercept, p will equal zero and so the focal length will be 0?
=> M+1= f/p
=> Mpp=f
But since it’s the y intercept, p will equal zero and so the focal length will be 0?