The boat travels 168 miles two ways. The trip downstream takes 6 hours and the the trip back takes 14 hours.
As the total distance traveled is 168 miles, the distance traveled in each direction is 84 miles.
Let the speed to the boat in still water be B and that of the current be C. When the boat goes downstream, its speed is B + C and when it goes upstream its speed is B - C
Time = distance/speed
=> 6 = 84/(B + C) and 14 = 84/(B - C)
=> B + C = 14 and B - C = 6
=> B = 10 and C = 4
The speed of the boat in still water is 10 miles/h and the speed of the current is 4 miles/h.
Its actually 168 miles downstream and 168 miles back, so 336 miles total. Not a total of 168 miles. I can see how thats confusing though because I got confused by that too.