= Solution
\b[The image is upright vertically when viewed down from behind, but left and right are interchanged.] To track the <image formation>, let $+x$ point forwards and $+z$ upwards. Incoming <light> travels along $-x$. The $45^\circ$ flip <plane mirror> sends it along $+z$. Before this <reflection>, the <optical lens> has inverted the scene's top into the $-z$ direction; the <law of reflection> maps that direction into $+x$ on the horizontal screen. The screen's far edge is the top of the picture for someone looking down from behind. The horizontal inversion from the <optical lens> remains, since this <plane mirror> does not change the lateral coordinate.
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