Lens question help!

https://isaacphysics.org/questions/two_lens_microscope?stage=a_level
Can you please help me answer part b?
I got that the focal length for a is 3 cm
And I have tried to use values and got 2.5 as the movement
I am a little confused by lenses does it mean that if it emerges as a parallel beam then the focal length is infinity therefore the reciprocal is 0?
Thank you so much!

... does it mean that if it emerges as a parallel beam then the focal length is infinity therefore the reciprocal is 0?

Any help would be amazing!

Helloo

Show how you get 2.5.



Focal length cannot be infinity.




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Sorry won't do it again
I am just a little confused (I haven't been taught by a teacher cause my school doesn't have a physics teacher so I am trying to teach myself)
Yh 2.5 is definitely wrong then
I think my confusion is that because the beams of light that are emerging are parallel to the lens then they will never meet at a focal point so how do I calculate the focal length if it does meet at a point
Thank you!