.
There is very useful trick, allowing solve such problems without using and/or memorizing complicated formulas.
The starting velocity was 12 m/s.
The ending velocity is 12 m/s + 3 s * 2 m/s^2 = 12 + 6 = 18 m/s.
The average velocity is half-the-sum = = 15 m/s.
The traveled distance is the average velocity times time (3 seconds)
15 m/s * 3 s = 45 meters.
ANSWER. The traveled distance is 45 meters.
Solved, completed and carefully explained.
This methodology works for any uniformly accelerated/decelerated movements.