Answer:
 4 mph
Step-by-step explanation:
The current subtracts from the boat speed going upstream, and add going downstream. The relation between speed, distance, and time is ...
  time = distance/speed
 2 1/4 = 12/(12 -c) + 12/(12 +c) . . . . . round trip time is the sum of the trip times
 9/4(12 +c)(12 -c) = 12(12 +c) +12(12 -c) . . . . multiply by (12 +c)(12 -c)
 144 -c^2 = 128 . . . . . . multiply by 4/9 and simplify
 16 = c^2 . . . . . . . . . add c^2 -128
 c = 4 . . . . . . . . . positive square root
The current was 4 mph that day.
