A diverging

lens with focus 5 cm is placed in a filament. Find the distance between the lens and the filament.

I use formula : $\displaystyle \frac{1}{f}=\frac{1}{di}+\frac{1}{do}$

I think the question is asking about do = distance of the object. But I don't know di (distance of image). How to solve this problem?

Thanks