Two dimensional acceleration question

F.B
My book stinks it doesnt explain or show you how to do enough examples. But anyways i have another problem now. I figured out how to do my other question. Anyways my question is.......

A car with a velocity of 25 m/s [E] changes its velocity to 25 m/s in 15s. Calculate the car's average acceleration.

How do i start this question off. I dont know what to do.

$$\vec{a}_{ave} = \frac{\Delta \vec{v}}{\Delta t}$$