Question 906532
upstream speed be x
downstream speed =2x

boat speed = y

upstream 
y-6=x............(1)

downstream
y+6=2x

y=2x-6
2x=y+6
x=(y+6)/2.................(2)

equate both
(y+6)/2 = y-6

y+6=2y-12
y=18
18km/h speed of boat