A parallel beam of rays from a distant source located on the optical axis falls on a thin negative lens. At a distance a behind the lens, perpendicular to its optical axis, there is a flat mirror. After the rays pass through the lens of reflection from the mirror and secondary passage through the lens, a virtual image is formed, located between the lens and the mirror at a distance of 3a/4 from the lens. Determine the focal length of the lens.

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