Final image distance using lenses

[kiwi101]

A 15cm focal-length converging lens is 18cm to the right of a 4.0cm focal-length converging lens. A 4.0cm tall object is distance to the left of the first lens.
For what value of is the final image of this two-lens system halfway between the two lenses?

My attempt:
I am working backwards:
s" = -9 (final virtual image displayed halfway between lenses)
f = 15
I solved for (1/s') + (1/s") = 1/f
s'= 5.6 cm (real image distance formed from lense 1)
f = 4cm
and now solve for s one more time and get s = 14
but that is wrong

 

[TSny]

I solved for (1/s') + (1/s") = 1/f
s'= 5.6 cm (real image distance formed from lense 1)

s' is object distance for 2nd lens. That's not the same as the image distance for the 1st lens. If the object of the 2nd lens is 5.6 cm to the left of the 2nd lens, how far is that to the right of the 1st lens?