When acceleration is constant, the average velocity is given by
where and are the final and initial velocities, respectively. By definition, we also have that the average velocity is given by
where are the final/initial displacements, and are the final/initial times, respectively.
Take the car's starting position to be at . Then
So we have
You also could have first found the acceleration using the equation
then solve for via
but that would have involved a bit more work, and it turns out we didn't need to know the precise value of anyway.