Question 1092062
Speed = distance/time 
Let s (speed) be the speed of the boat and c (current) the speed of the current
:
S(upstream) = s-c
S(downstream) = s+c
~~~~~~~~~~~~~~~~~~~~~~~~~~
Upstream data:
d = 18
t = 6
18/6 = 3km/h <---- this is the speed of the boat going upstream.
. . . . . . . . . . . . . . . . .
Downstream data:
d = 18
t = 3
18/3 = 6km/h <--- this is the speed of the boat going downstream.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Summarizing:
S(up) : s-c = 3km/h
S(dwn): s+c = 6km/h
. . . . . . . . . . . . . . . . . . . . 
Let's find the speed of the current:
S(up) : s = 3+c
S(dwn): s = 6-c
3+c = 6-c
2c = 3
c = 1.5 <-- this is the speed of the current.
Substitute in the equation to find the speed of the boat in still water:
3+1.5 = 6-1.5 = 4.5 <--- this is the speed of the boat in still water.