Focal length of the lens

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Homework Statement
An object is in a distance of 14 cm from a converging lens. The lens produces a virtual image at a distance 18 cm from the object.

What is the focal length of the lens (in cm).
Relevant Equations
1/f = 1/di + 1/do
1744709958617.png

I attempted and keyed in the answer as 63 but I was graded wrong. What am I missing?
 
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syllll_213 said:
Homework Statement: An object is in a distance of 14 cm from a converging lens. The lens produces a virtual image at a distance 18 cm from the object.
 
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