Calculating Car Speed After Sudden Braking

[012435]

Homework Statement

driver slams breaks. car slows with acceleration of -5.6m/s^2 for 4.20s for 62.4m.

Homework Equations

with wat speed does the car then strike an object?

The Attempt at a Solution

i thought of using equation Xf=Xi+Vi(t)+1/2at^2
i am not sure if this is the best choice but when using this i am getting stuck with the initial velocity. am i in the right track

 

[learningphysics]

Yes, you can use that equation. Find vi, then you can find vf...