Kinematics Problem: Train braking and blocking an intersection

[Zacharrry]

Homework Statement

A train 400 m long is moving on a straight track with a speed of 21.6m/s. The engineer applies the brakes at a crossing, and later the last car passes the crossing with a speed of 5.0m/s. Assuming constant acceleration, determine how long the train blocked the crossing. Disregard the width of the crossing.

Homework Equations

One of the "big four" kinematics equations (velocity, initial velocity, time, acceleration, etc.)

The Attempt at a Solution

I tried the problem and got 37.04 seconds, but it told me it was incorrect. I am not sure what I did wrong, so can anyone help by telling me what they got and/or how to solve it?

 

[Merlin3189]

You need to show what you did, which of the big four formulae you used, what numbers you put in, etc.