Find Focal Length of Mirror with 9.5 Magnification, 14mm from Object

[Ry122]

A mirror is required with magnification 9.5 when an object is 14mm from the mirror
What is the focal length

My attempt:
M=v/u
9.5=v/14
v=133

1/f = (1/v)+(1/u)
1/f=(1/133)+(1/14)
f=12.67

Since no object is magnified when the created image is outside of 2f
this answer can't be correct.

 

[Bill Foster]

Try this equation:

$$M = - \frac{S_2}{S_1} = \frac{f}{f - S_1}$$

See http://en.wikipedia.org/wiki/Lens_(optics) for more info

 

[Bill Foster]

The answer I get is 15.65.

 

[Ry122]

2x15.65 is still less than 133