Two people are rowing a boat at a speed of 9 m/s. in 3 seconds, they dock the boat at a pier. what is the boatâs acceleration?

Acceleration is a measure of how fast a subject or object has changed its speed over a course of time. This parameter may take a positive value if the speed is increased and negative if the speed is decreased. The latter is rather called deceleration.
With this being said, the acceleration is calculated by dividing the speed by the given time. That is,

acceleration = speed / time

Substituting the known values,
 
acceleration = (9 m/s) / (3 s) 
acceleration = 3 m/s²

Since the boat is docked this means that the speed is reduced. The value then becomes -3m/s². 

Therefore, the boat in this item has an acceleration of -3 m/s². 

You get 6 for mechanical