Focal length of lens

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Hello, I am a bit confused about what the image distance is in this question. And how to find the focal length?

Thank you!

Below is a typical ray diagram for the set-up you have as a problem.

Can you see “why” the focal length is shown as what it is?

Ohh, is it because when the image formed exactly on the object, it can only happen when the light rays between the lens and plane mirror are parallel, causing the reflected rays to pass along the incident rays, meaning the object must be placed at the focus?

Indeed, a good explanation. 😊
Some additional points, the plane mirror is used to “simulate” light coming from far away, such that the light rays can be taken to be parallel.
Light rays parallel to the principal axis passing through the converging lens will meet at a point called the principal focal point which is one focal length from the optical centre.