Acceleration is the rate of change of velocity in time. It asks for average acceleration. The formula of average acceleration is:
you can use this formula only for average acceleration, don't even think of using it for instantaneous acceleration.
So, we have to find , which means the change of velocity. Remember that velocity is a vector. We have to find it using vector subtraction. You can't say . This only would be true if the velocities were in the same direction. In here, they have different angles.
O.K, here are the velocity vectors.
I leave you the subtraction work. After doing that, find the velocity of the subtracted vector, this will be our . You know . So use them in the average acceleration formula to finish.