Why is the image distance negative in a Two Thin Lens setup?

[whitehorsey]

1. Two 25.0 cm focal length converging lenses are placed 16.5 cm apart. An object is placed 35.0 cm in front of one lens. Where will the final image formed by the second lens be located?

I got the right answer but I'm not sure why the image distance is negative when the image (formed by the first lens) is to the right of the converging lens.
2. 1/f = 1/o + 1/i, where f = focal length, o = object distance from lens, i = image distance from lens.
3. I've done all the steps but I'm just not sure why the image distance is negative for the image formed from the first lens.

Here is my work:
1/f = 1/o + 1/i
1/25 = 1/35 + 1/i
i = 87.5 cm to the right of lens 1, which is 71 cm to the right of lens 2.

However, the answer to the image distance (from the first lens) was -71 cm, not +71 cm.

Can you explain this to me please?

 

[grzz]

It is best explained by a diagram.