Answer:
~ 0.001 m/s^2
Explanation:
If you convert 12 miles per hour to, miles per second, it makes this easier to calculate, so do that first
12m/ 1 hour * 1 hour/ 60 mins * 1 min/ 60 sec = ~0.003333 miles per second.
Next remember that acceleration = velocity/time
So our new velocity: 0.003333 over time: 3 seconds = the acceleration.
0.003333/3 = 0.001 m/s^2