Remember that rate times time gives you distance. Let v be the speed of the boat in still water and let w be the speed of the current. When the boat moves downstream, it will move at v + w relative to the shore because it’s moving with the current. When it moves upstream, it’ll travel at v – w because it’s fighting against the current. We know that it moves 5 km upstream in the same time as it moves 17 km downstream. So, solve

vt = d

for time:

t = v/d

Then we know that

(v + w)/17 = (v-w)/5

w is 6, so

(v + 6)/17 = (v – w)/5

5(v + 6) = 17(v – 6)

5v + 30 = 17v – 102

12v = 132

v = 132/12 = 11

So the boat moves 11 kph in still water.

As a check: upstream, it’ll move 11 – 6 = 5 kph, so it’ll take an hour to go 5 km.

Downstream, it’ll move 11 + 6 = 17 kph, so it’ll take an hour to go 17 km, and we’ve got it.

17k/h-6k/h= 11k/h

you can check that by

5k/h+6k/h=11k/h