Question about images formed by double thin lenses?

I have a hard time figuring out how to work with a combination of 2 camera lenses. L1 focal length f1=15cm and L2 has f2=13cm which project image onto a film 12cm from the first lens. Distance of L2 which is closer to the film can vary between 5cm and 10cm from the film. I am trying to find the range of distances in front of L1 for which objects can be focussed on the film. What method to follow please ? I just know the lens equations.