Question 540824
Let the speed of the boat in still water be x.
The speed of the boat upstream (in kph) is x-3
The speed of the boat downstream (in kph) is x+3
and we know its twice as fast as when going upstream, so
x+3=2(x-3) --> x+3=2x-6 
subtracting x from both sides we get
3=x-6
adding 6 to both sides we get 9=x, or more conventionally written
x=9
The speed of the boat in still water is 9 kph.