Kinematics Question!

1. In a crash simulation, a car traveling at x m/s can stop at a distance d m with a maximum deceleration. If the car is traveling at 2x m/s:

A. Stopping time is doubled
B. Stopping distance is doubled
C. Stopping distance is quadrupled
D. Both A and C

Ok, so I thought this was a relatively easy question. However, I managed to butcher it. My problem is, I do not understand which equation to use. To find if the distance doubled or quadrupled I used the equation: d = ((Vi + V)/2) * t. Since the car will be stopping the V will be zero and you simply get d = (Vi/2) * t. If the velocity is doubled, distance is doubled. However, the book used the equation V^2 = Vi^2 + 2ad. Thereby getting, stopping distance quadrupled.

Also for time they used V = Vi + at.
Answer is D.

Once again, I am lost to know which equation to use. Since both seemed to work..
Thanks for your help!

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