Question 162845
Let x be the speed in still water.
Then the downstream speed relative to land is x + 6, and
the upstream speed relative to land is x - 6.

the distance traveled downstream = 3(x + 6)
the distance traveled downstream = 5(x - 6)

Now use the following relationship to set up an equation:
the distance traveled downstream = the distance traveled upstream.

So
3(x + 6) = 5(x - 6)
Solving the equation for x, we have
3x + 18 = 5x - 30
48 = 2x
24 = x
Thus x = 24
So the speed in still water is 24 km/h