Let

t1 = the time of the upstream trip

t2 = the time of the downstream trip

d = 6 km the distance traveled in each direction

v = the speed of the boat in still water

c = 5 km/h the speed of the current

The speed upstream is: v - c

The speed downstream is: v + c

Since speed = distance/time => distance = speed*time we have:

(v - c)*t1 = d

(v - 5)*t1 = 6

(v + c)*t2 = d

(v + 5)*t2 = 6

The time for the round trip is 9 hrs:

t1 + t2 = 9

For (v - 5)*t1 = 6 we solve for t1: t1 = 6/(v - 5)

For (v + 5)*t2 = 6 we solve for t2: t2 = 6/(v + 5)

Plug into t1 + t2 = 9:

6/(v - 5) + 6/(v + 5) = 9

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click here to see the equation solved for v

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The speed of the boat in still water is 5.71 km/h.