let

v = 7 km/h the speed of the boat in still water

d = 40 km the distance traveled in one direction

t1 = the time of the travel upstream

t2 = the time of the travel downstream

t1 + t2 = 14 hr the total time of the travel

s = the speed of the stream

traveling upstream the speed of the boat is: v - s = 7 - s

traveling downstream the speed of the boat is: v + s = 7 + s

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

t1 + t2 = 14

d/(v + s) + d/(v - s) = 14

40/(7 + s) + 40/(7 - s) = 14

by solving we find:

s = 3 km/h

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

the speed of the stream is 3 km/h.