Without his glasses, Isaac can see objects clearly only if they are less than 4.7 m from his eyes. Required:
What focal length glasses worn 2.1 cmcm from his eyes will allow Isaac to see distant objects clearly?

The focal length is  

Explanation:

From the question we are told that

  The  image distance  is   the negative means  image is formed behind the lens

Generally the object distance is   this mean the light ray coming from the object are parallel to each other

So from lens equation

=>  

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