We have a container which we fill 9 gallon of water.
Take 7 gallon full of water, pour in to the 11 gallon. Take again 7 gallons with the 7 gallon container, pour into the 11 gallon container, till it is full. So 3 gallons are remaining in it now. Pour it into our container, where you want 9 gallons.
Empty the 11 gallon container. Repeat the above procedure two more times.
Then you get 9 gallons.