1 (the job) = x * 72 where x is the rate each man works.

1/72 = x = the amount of the wall each man builds per day.

1/72 * 12 = 12/72 of the wall built (by 12 men) each day. We need a total of 72/72 (1) wall.

72/12 = 6 days for the 12 men to build the wall.

You can also look at it simple. 12 men complete the wall twice as fast as 6 men. They complete the job in half the time. So 12 men complete the wall in 12/2 = 6 days.