If a river is flwoing downstream at a velocity of 2.5 m/s how fast would a kayak be paddled? distance is 4500m
That depends upon who is doing the paddling, and in which direction.
Surely that is not the entire question you were supposed to answer. More facts are required.
When yu say "how fast?" are you really asking how long it would take? Otherwise it makes no difference what the distance is.