First and foremost: no need for "CMD /k"! Why "CMD.EXE", not "PowerShell.EXE" or something like that? Those are just application, exactly as any other application, nothing "special" or "system" about it. You could write anything like that by yourself.
No, you need to execute ffmpeg itself and nothing else:
"ffmpeg.exe", @"-b:10000k -r25 -an -i concat:""C:\Video conversion\Original\Cam06[21_48_59-22_29_19].avi|C:\Video conversion\Original\Cam06[22_29_19-23_36_00].avi"" ""C:\Video conversion\test.avi""");
(I'm not sure your ffmpeg syntax is correct though. Check it manually first.)
And of course, there is no cases when hard-coded path names can be useful. All path names should be calculated during runtime based on user-supplied data, configuration data, environment, etc.