Triangle Trigonometry and Vectors · Superchallenge problem

A river is $300$ m wide with straight parallel banks, and its current flows at $5$ km/h. A boat can move at $3$ km/h relative to the water, which is slower than the current, so it will be carried downstream however it is steered. If the boat keeps a constant heading chosen to make the downstream drift as small as possible, how far downstream, in meters, does it land on the far bank?