Answer:
-4 m/s
Step-by-step explanation:
The maximum speed of the river relative to the shore is 3 m/s. Its velocity is +3 m/s
The maximum speed of the freighter relative to the river is 7 m/s since it moves in the opposite to direction to the flow of the river, its velocity is -7 m/s.
The velocity of the freighter relative to the shore = velocity of the freighter relative to the river + velocity of the river relative to the shore = -7 m/s + 3 m/s = -4 m/s