A candle is placed in 15.0 cm in front of a convex mirror. When the convex mirror is replaced by a plane mirror, the image moves back 7.00 cm (further away from the mirror). Fing the focal length of the convex mirror.

I'm not sure which one, I never did mirrors really, but I think it would be either 8/15 or 15/8

When the plane mirror is there, the image is, by definition, 15cm behind the mirror, and it moves forward 7cm to 8cm behind the mirror when you put the convex one in...

You will need to apply your knowledge but I hope this gets you started :)