Answer:
The question seems to be looking for vf, which may be the final velocity.
We know that the car stops, so the car is at rest, this means that the final velocity is equal to zero.
vf = 0 m/s
Now, we also can find the acceleration, the velocity equation can be written as:
v(t) = a*t + 12m/s
We know that in 3 seconds the car stops, so we have:
v(3s) = 0 = a*3s + 12m/s
a= -12m/s*(1/3s) = -4m/s^2
So in those 3 seconds where the car is stoping, the acceleration of the car is -4m/s^2, and the velocity of the car in those 3 seconds can be described by the equation:
v(t) = -4m/s^2*t + 12m/s
