How to find the final velocity o

[Jessica Lewis]

The question is:
a bullet that is shot from a target is traveling at an initial speed of 80 m/s and it is 6 m away from its target. The target is wearing a bullet proof vest that is 0.26 m thick. As the bullet makes contact with the vest is slows down at a rate of 9000 m/s2. What is the final velocity of the equation?I would assume that the equation used would be Vf^(2) = Vi^(2) + 2ad, however each time I attempt this question I get an answer that makes absolutely no sense.
So, please help me,
Thank you! :)

 

[toboldlygo]

EDIT: I realized I did the problem and, according to the guidelines, that isn't allowed! So, instead: for the acceleration, is 9000 m/s² constant for the whole 6.26 m?

 

[haruspex]

each time I attempt this question I get an answer that makes absolutely no sense.
So, please help me,
Thank you! :)

how do you expect anyone to tell you where you are going wrong if you do not post your working?