How would you solve this physics problem. I am given Vf, distance and time. Unknown is Vi

Question

Car is braking to a stop. I would assume constant acceleration.

Answer 1

vf^2 = vi^2 + 2 ax

at constant acceleration a = 0 ... implies vf and vi are the same

Answer 2

Constant acceleration certainly does not mean a = 0. You would however use Vf^2 = Vi^2 + 2ax  

If the car is braking to a stop, make sure you make the acceleration negative and of coarse your final velocity must be 0 if the car is coming to a stop.  

Answer 3

An alternative formula for constant acceleration problems like these:

d = (average velocity) * time = (Vi + Vf)/2 * t

(Vi * t) / 2 = d - (Vf * t) / 2

Vi = 2 * [d - (Vf * t)/2] / t

and Vi is isolated so now it's just number-crunching