I'm not sure what "1.5km downstream" means. Is it 1.5KM relative to Point A, or what?
As for the actual problem: The boat is moving from point A to B, at a speed of 20km/h. We can represent its vector as AB=20. The vector parallel to the river bank we can say is I dunno RC (river current), and is equal to 12. If the captain takes his boat from point A to B, he will be pushed off course away from B, by the current, since if you add AB to RC, you get a vector ABRC that is equal to the actual direction and magnitude of the boat. The captain should then direct the boat to the right of Point B, in order to get a total vector that points from A to B.