a-maths!!!!!!!

Let the speed of the boat in still water be x km / h.
In the upstream journey, the speed of the boat: ( x – 3 ) km / h
In the downstream journey, the speed of the boat: ( x + 3 ) km / h
Then,
36 / ( x - 3 ) – 36 / ( x + 3 ) = 1
36 ( x + 3 ) – 36 ( x -3 ) = ( x + 3 )( x – 3 )
36 ( x – 3 – x + 3 ) = x2 – 9
216 = x2 – 9
x2 = 225
x = 15 or –15 ( rejected )