Explanation:

Speed of man in still water, x = 9/2 km/hr
Let speed of stream = y km/h
Downstream speed = (9/2 + y)
Upstream speed = (9/2 - y)
 According to the question,
2 [Distance/(9/2 + y)] = Distance/ (9/2 - y)
2/ (9/2 + y) = 1/(9/2 - y)
(2 × 2) /(9 + 2y) = 2/(9 – 2y)
2/(9 + 2y) = 1 / (9 – 2y)
 18 – 4y = 9 + 2y
 6y = 9
y = 9/6 = 3/2 = 1.5 km/h