Diverging and Converging Lenses

[slaw155]

Homework Statement

A diverging lens (f = –13.0 cm) is located 18.0 cm to the left of a converging lens (f = 31.0 cm). A 3.30-cm-tall object stands to the left of the diverging lens, exactly at its focal point.
Determine the distance of the final image relative to the converging lens

Homework Equations

1/f = 1/v + 1/u

The Attempt at a Solution

1/-13 = 1/13 - 1/v
1/v = -2/13
v= -6.5
The substituting that v value into lens maker formula again for converging lens using f=31
6.5+18 = 24.5cm meaning the image of first lens is the object for second lens which is 24.5cm away from the second lens.
1/31 = 1/24.5 + 1/v
This gives a v value which is negative which is wrong for a converging lens.
What have I done wrong? How do I correctly solve the problem?

Thanks

 

[vela]

The image distance for a converging lens isn't always positive.