separation between the lenses?

What is the separation between the lenses?

Question:

Fundamental lens formula:

The fundamental lens formula is given:

$\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}$

where $f$ is the focal length of the lens. $d_o$ is the distance of the object from the lens and $d_i$ is the distance of the image from the lens.

The magnification of the lens $M$ is given:

A converging lens with a focal length of 4.0 cm is to the left of a second identical lens.

When a feather is placed 12 cm to the left of the first lens, the final image is the same size and orientation as the feather itself.