how to convert a 3d 50% sbs mkv to 2d mkv with avidemux

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convert a 3d 50% side by side .mkv file to a 2d one, if you have approx 2+ hours to not use your computer.. the software used is Avidemux http://www.avidemux.org contains a download link but who knows where the 50% 3d .mkv came from...