Solving for average acceleration.

[Polyrix]

Homework Statement

A car initially moving at 12 m/s [E 30^o N] accelerates until it reaches a final velocity of 10 m/s [E]. What was the average acceleration of the car?

I couldn't attempt anything. There isn't any magnitude given save for velocity, so I have no clue how to solve this, and whether it's even possible.

 

[Raghnar]

you can't tell the avarage acceleration if you don't know in how much time the car decelerate from 12 to 10 m/s.