Answer: -0.0288 m/s^2
Explanation:
Let's suppose that the ferry decelerates at a constant rate A (deceleration is an acceleration in the opposite direction to the original motion of an object)
Then the acceleration equation of the ferry will be:
a(t) = -A
(the negative sign is because this acceleration is in the opposite direction with respect to the movement of the ferry)
To get the velocity equation of the ferry, we need to integrate with respect to the time, t, we will get:
v(t) = -A*t + v0
where v0 is the initial velocity of the ferry, v0 = 12m/s.
v(t) = -A*t + 12m/s
For the position equation of the ferry we need to integrate again over time:
p(t) = (-A/2)*t^2 + (12m/s)*t + p0
Where p0 is the initial position of the ferry, in this case, it can be zero, because it will depend on where we put the origin on our coordinate axis.
then p0 = 0m
P(t) = (-A/2)*t^2 + (12m/s)*t
The ferry will come to rest at the moment when it's velocity is equal to zero, this will happen when:
v(t) = 0m/s = -A*t + 12m/s
We need to find the value of t.
A*t = 12m/s
t = (12m/s)/A
Now we can replace this in the position equation because we know that the ferry needs 2.5 km or 2500 meters to come to rest.
p( (12m/s)/A) = 2500m = (-A/2)*( (12m/s)/A)^2 + (12m/s)*((12m/s)/A)
2500m = (-72 m^2/s^2)/A + (144m^2/s^2)/A
2500m = (72 m^2/s^2)/A
2500m*A = (72 m^2/s^2)
A = (72 m^2/s^2)/2500m = 0.0288 m/s^2
and the acceleration of the ferry was -A, then the acceleration of the ferry is:
-0.0288 m/s^2
to 
(from (-a/2) to
(a/2))