By Pythagoras AB^2 = 12^2 - 6^2 = 108
AB = sqrt 108 = 10.39 to nearest hundredth
The perpendicular from M to DC will be parallel and equal to AB so it = 10.39.
Also AB^2 = CB * DB
108 = 6 * DB
DB = 108/6 = 18
so DC = 18-6 = 12
MD^2 = (1/2*12)^2 + 108 = 144
so MD = 12