What is the focal length of the lens?

[keemosabi]

Homework Statement

When a diverging lens is held 12 cm above a line of print, as in Figure 26.29, the image is 4.1 cm beneath the lens. What is the focal length of the lens?

Homework Equations

1/di + 1/do = 1/f

The Attempt at a Solution

1/di + 1/do = 1/f
1/4.1 + 1/12 = 1/f
f = 3.0559 cm

What did I do wrong?

 

[Redbelly98]

The image is virtual, so di is considered to be negative.