A boat leaves one bank of a river and heads straight across, due north, at 4 m/s relative to the…

Step 1. The sum. $\vec v = 3\hat i + 4\hat j$ metres per second.

Step 2. The speed. $\lvert\vec v\rvert = \sqrt{9 + 16} = 5$ metres per second.

Step 3. The angle. Measured from north, $\tan\theta = \dfrac{3}{4}$, so $\theta \approx 36.87^\circ$.

Step 4. The time. The boat crosses $200$ metres northwards at $4$ metres per second, so the time is $\dfrac{200}{4} = 50$ seconds.

Step 5. The drift. The current carries the boat $3 \times 50 = 150$ metres east.