How to Calculate Focal Length of a Convex Mirror?

[matt72lsu]

Homework Statement

The virtual image produced by a convex mirror is one-quarter the size of the object.
b) What is the focal length of this mirror?

Homework Equations

1/di = 1/f - 1/do

The Attempt at a Solution

i found di to be -9.3 cm but when i solved for f i keep getting it wrong. Help! (and -9.3 is correct so that's not the problem)

 

[ideasrule]

Can you show your calculations?

 

[matt72lsu]

i did 1/-9.3 = 1/f - 1/37 (do) then solved for f