Question 1196739
The other tutor got the right formula but made a mistake
In substitution.

Acceleration (a) is the change in velocity (Δv) over the change in time (Δt), represented by the 
equation a = Δv/Δt. 

Where ∆v = V2 - V1
      ∆t = t2 - t1

This allows you to measure how fast velocity changes in meters per second squared (m/s^2). 

So we have,
a = (5-15)/5-0
a = -2

Answer:
The boat's average acceleration is -2m/s^2.
Negative number means the boat is decelerating 
aat 2m/s^2.