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In the right triangle, we have: .
Relative to the ship, he is walking 0.749 m/sec forward.
Since the ship is moving forward at 5 m/sec,
. . the man is moving forward at 5.749 m/sec (relative to the water).
(That's where they got that number.)
So relative to the water, his movement looks like this:
* - - - - - *
1.854 | /
We have: .
Relative to the forward direction of the boat, his angle is about 17.9°.