Determining velocity after acceleration over a certain distance?

A car passes you at 18 m/s to the north and increases its speed at a rate of 3.0 m/s^2. Determine the car's velocity when it has accelerated for 50m.

I used the formula Vf^2 = Vi^2+2ad and my answer was more then 15m off the final answer.

What am i doing wrong !?
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