Theia said:
I try to add a picture a bit later. Need to figure out first how it's done...
//To be added: I draw this picture to help my solving process (click to see it fully):
Here you go. (Happy)
[TIKZ]
\newcommand\anglephi{atan(3/2)}
\newcommand\anglealpha{(90-atan(3/2))/2}
\coordinate[label=below right:O] (O) at (0,0);
\coordinate[label=left:A] (A) at (-3,0);
\coordinate[label=above:B] (B) at (2,3);
\coordinate[label=right:C] (C) at (6,0);
\coordinate[label=below:D] (D) at (-3,-4.5);
\coordinate[label=left:A'] (A') at ({180-\anglealpha}:{3*cos(\anglealpha)});
\coordinate[label=above:B'] (B') at ({\anglephi+\anglealpha}:{sqrt(2^2+3^2)*cos(\anglealpha)});
\coordinate[label=right:C'] (C') at ({-\anglealpha}:{6*cos(\anglealpha)});
\coordinate[label=below:D'] (D') at ({180+\anglephi+\anglealpha}:{sqrt(3^2+4.5^2)*cos(\anglealpha)});
\draw[thick] (O) node at +({\anglephi/2}:1) {$\phi$} +(0.6,0) arc (0:\anglephi:0.6);
\draw[thick] (O) node at +({\anglephi + \anglealpha/2}:1.4) {$\alpha$} +({\anglephi}:1) arc (\anglephi:\anglephi+\anglealpha:1);
\draw[thick] (O) node at +({180 - \anglealpha/2}:1.4) {$\beta$} +({180 - \anglealpha}:1) arc (180-\anglealpha:180:1);
\draw[rotate={-90-\anglealpha}] (A') rectangle +(0.3,0.3);
\draw[rotate={-90-\anglealpha}] (B') rectangle +(0.3,0.3);
\draw[rotate={90-\anglealpha}] (C') rectangle +(0.3,0.3);
\draw[rotate={90-\anglealpha}] (D') rectangle +(0.3,0.3);
\draw[rotate={-90-\anglealpha}] (A') ++(B') rectangle +(0.3,0.3);
\draw[rotate={90-\anglealpha}] (C') ++(D') rectangle +(0.3,0.3);
\draw (A) -- (C);
\draw (B) -- (D);
\draw (A') -- (C');
\draw (B') -- (D');
\draw[blue, ultra thick] (A) -- (B) -- (C) -- (D) -- cycle;
\draw[red, ultra thick] (A') -- +(B') -- (B') -- +(C') -- (C') -- +(D') -- (D') -- +(A') -- cycle;
[/TIKZ]