Answer:
60 m
Step-by-step explanation:
The boat has two separate motions:
1- A motion due north, with constant speed of 10 m/s
1- A motion due east, due to the current, at speed of 2 m/s
We know that the river is 300 m wide, so we can consider first motion 1) to find how much does it take for the boat to cross the river:

Now we can find how far downstream the boat moved by calculating the distance that the boat covered moving east during this time interval:
