Let the car is moving with velocity u ft/second. And final velocity of the car is v ft/second. After appllying the break the deceleration f of the car is given by f=-40 ft/second^2.
As when the car comes to a stop, v=0.
Now we know that v^2=u^2+2fs.
Where s is the distance travelled by the car.
or, s=u^2/80 ft.
Means after moving by u^2/80 ft. the car comes to a stop.