The car's speed was zero at the beginning of the 12 seconds, and 18 m/s at the end of it. Since the acceleration was 'uniform' during that time, the car's average speed was (1/2)(0 + 18) = 9 m/s.
12 seconds at an average speed of 9 m/s ==> (12 x 9) = 108 meters .
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That's the way I like to brain it out. If you prefer to use the formula, the first problem you run into is: You need to remember the formula !
The formula is D = 1/2 a T²
Distance = (1/2 acceleration) x (time in seconds)²
Acceleration = (change in speed) / (time for the change) = (18 m/s) / (12 sec) = 1.5 m/s² .
Distance = (1/2 x 1.5 m/s²) x (12 sec)² = (0.75 m/s²) x (144 sec²) = 108 meters .
