Finding focal length given magnification and object distance?

[speedrsr]

The distance between an object and its upright image is 15.5 cm. If the magnification is 0.7, what is the focal length of the lens being used to form the image?

Answer in units of cm.

So I tried combining the equations p+q=15.5 and -q/p=.7 and still was unable to come up with a correct answer.. How exactly do can you find the focal length with only the magnifaction and distance?

 

[Stonebridge]

For there to be a diminished, upright image we are looking at a concave/diverging lens.
The virtual image is on the same side of the lens as the object. It is also nearer the lens. This can be shown on a ray diagram.
This gives q/p =0.7 and p-q = 15.5
Solve for p and q