15+3=18km/hour
Think about it like this. The boat is going 15 faster than the river, and the river is going 3 faster than the bank, so the boat is going 18 faster than the river bank
Is the divine injunction found in Genesis 1:28, in which God, after having created the world and all in it, ascribes to humankind the tasks of filling, subduing, and ruling over the earth.