Optics question

[ms. confused]

Hi, could someone tell me what they get for the focal length of this question:

A diverging lens produces an image 10.0cm from the lens when the object is placed 30cm from the lens. Calculate the focal length and magnification of the lens.

*I got -7.5cm, but apparently it's supposed to be -15cm. What am I doing wrong? :surprised

[daster]

si=-10
so=30

1/so + 1/si = 1/f
1/30 - 1/10 = 1/f
(10-30)/(300) = 1/f
-1/15 = 1/f
f=-15cm