Hello,I dont know how to do this short exercise,I mean it does not make any sense.The only thing i know is that the answer is 3.3cm but i cant do the procudure finding the same result.This is the exercise:
A concave mirror makes a 3 cm image of a 6 cm object when the object is placed 10 cm from the mirror. What is the focal length of the mirror?

It really would help me if you lend me a hand on this.
many thanks