An approaching storm is moving at 15 km/hr. what do you need to know to determine its velocity?

A DIRECTION
Velocity is a vector quantity

In this question, due importance should be given to all the information's that are provided in the question. Based on those information's the answer to the question can be easily deduced. Let us first write down all the provided information's.
Initial Velocity (V1) = 0 m/s
Final Velocity (V2) = 5 m/s
Initial time (T1) = 0 sec
Final Time (T2) = 4 sec
Then
Average acceleration of the car = (V2 -V1)/(T2 - T1)
                                                   = (5 - 0)/(4 - 0) m/s^2
                                                   = 5/4 m/s^2
                                                   1.25 m/s^2
So the average acceleration of the car over this period of time is 1.25 m/s^2. I hope the process is clear for you to understand.