[tex]\mathcal{P}_{b} = 3\ell = 18\sqrt3 |_{:3} \Longrightarrow \ell = 6\sqrt3\ cm
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\mathcal{P}_{ABB'A'} = 2\ell + 2m = 28\sqrt3 \Rightarrow 2\cdot6\sqrt3+2m = 28\sqrt3|_{:2} \Rightarrow
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\Rightarrow 6\sqrt3+m = 14\sqrt3|_{-6\sqrt3} \Rightarrow m = 8\sqrt3\ cm[/tex]