Answer:
B. 73.19 mi/hr
Explanation:
First we need to define the angles from a common reference. Robin's watercraft is moving 75° west of south, which is the same as 15° south of west. The river is flowing at 75° south of west.
The watercraft velocity relative to the water is:
wx = 70 cos 15° ≈ 67.615
wy = 70 sin 15° ≈ 18.117
The river's velocity is:
rx = 6 cos 75° ≈ 1.553
ry = 6 sin 75° ≈ 5.796
Robin's net velocity is:
vx = wx + rx ≈ 69.168
vy = wy + ry ≈ 23.913
The speed (magnitude of velocity) is:
v = √(vx² + vy²)
v ≈ √(69.168² + 23.913²)
v ≈ 73.19