let

v = the speed of the boat in still water

d = 6 km the distance traveled in one direction

t1 = the time of the travel upstream

t2 = the time of the travel downstream

t1 + t2 = 5 hr the total time of the travel

s = 3 km/hr the speed of the stream

traveling upstream the speed of the boat is: v - s = v - 3

traveling downstream the speed of the boat is: v + s = v + 3

since speed = distance/time follows time = distance/speed

t1 + t2 = 5

d/(v - s) + d/(v + s) = 5

6/(v - 3) + 6/(v + 3) = 5

by solving we find:

v = 4.43 km/hr

click here to see the step by step solution of the equation:

the speed of the boat in still water is 4.43 km/hr.