Optics question, locate mirror and its focal length

[SlothNast]

Homework Statement

"An object 10 cm tall is placed at the zero mark of a meter stick. A spherical mirror located at some point on the meter stick creates an image of the object that is upright, 4 cm tall, and located at the 42 cm mark of the meter stick. Where is the mirror located and what is the focal length?"

Homework Equations

1/do + 1/di = 1/f

The Attempt at a Solution

 

[voko]

What have you done so far?

 

[SlothNast]

Well I've determined it must be a convex mirror, for the upright and miniature image. I have drawn a few rays from the real 10 cm object to the 4 cm image, but I am not sure how to find the location of the mirror and its focal point from that.

 

[voko]

What rays in particular have you tried? Just two rays is enough.