Answer:
11 shirts
Step-by-step explanation:
Since 12 of the 45 minute are devoted to setup, the actual printing time is 33 minutes. At 3 minutes per shirt, that's enough time for ...
(33 min)/(3 min/shirt) = 11 shirts
A batch consists of 11 shirts.
_____
Alternate solution
Another way to consider this is to realize that the setup time is the time required to print 4 shirts. 45 minutes would be enough time to print 45/3 = 15 shirts, but the time for 4 shirts is spent setting up. That leaves the batch size at 15 -4 = 11 shirts.
