As we can see, angle DCF is made up of 2 different angles, DCE and ECF, the sum of the measure of these two angles is equal to the measure of angle DCF.
In 3 hours, it prints 500 shirts. That means 6 hours it will print 1,000 shirts, in 9 hours it will print 1,500 shirts, and in 12 hours, it will print 2,000 shirts. So in 12 hours, it will print 1,800 shirts.