let b = the speed of the boat in still water and c = the speed of the current. now we have;
during the trip downstream b + c = 336 / 12 = 28 --->>> b+c = 28 mph (1)
during the trip back b - c = 336 / 14 = 24 --->>> b - c = 24 mph (2)
(1) + (2) = 2b = 52 so b=52/2 --->>> b=26 mph (1)-(2) = 2c = 4 c=4/2 c = 2 mph = current speed
speed of boat in still water; from (1) we get b=28-2=26 mph speed in still water
of caurse the same result if we take the (2) we get b-c=24 -->> b=24+2=26 mph = speed in still water