Answer: The speed of the current is 3.5 km/h.
Step-by-step explanation:
When they move in the river, the speed of the canoe is:
Sc is the speed of the canoe and Sr is the speed of the current.
Sc + Sr (when they move in the same direction as the current)
Sc - Sr (when they move in the opposite direction as the current)
So we have that in one hour, they moved 2km and then they returned other 2km.
Now, using the relation:
Velocity*time = distance
we can write 1hour = T1 + T2 such that:
(Sc + Sr)*T1 = 2km
(Sc - Sr)*T2 = 2km
T1 + T2 = 1hour -------> T2 = 1h - T1
Sc = 6km/h.
So we can write this as two equations:
(6km/h + Sr)*T1 = 2km
(6km/h - Sr)*(1h - T1) = 2km.
Now, the first step will be isolate one of the variables in one equation, we can get:
T1 = 2km/(6km/h + Sr)
and replace it in the second equation:
(6km/h - Sr)*(1h -2km/(6km/h + Sr)) = 2km.
Now we solve this for Sr, i will ignore the units, so it is easier to read the math.
(6 - Sr) - 2*(6 - Sr)/(6 + Sr) = 2
(6 - Sr)*(6 + Sr) - 2*(6 - Sr) = 2*(6 + Sr)
6^2 - Sr^2 - 12 + 2*Sr - 12 - 2*Sr = 0
-Sr^2 + 36 -12 -12 = 0
-Sr^2 + 12 = 0
Sr = √12 = 3.5km/h
(where we take the positive value of the square root)
. The first term reduces to 
, the second to 
, and the third to 3. Putting them all together with the signs in between we have 