With the previous two problems in mind, imagine a selfluminous object on the central axis of a

Question:

With the previous two problems in mind, imagine a selfluminous object on the central axis of a thin positive lens. The object is a distance d from the screen on which the image appears. Now suppose the lens is moved toward the object to a new location, whereupon the image on the screen is N times larger than it was originally. Show that the lens has a focal length given by

VNd f = 1 + VN)? 2