Just interested why you figured out v2 - v1, instead of v2+v1

Surely the question is just asking you to find the average acceleration (otherwise it would be like.."Graph the acceleration")

Now, i'm not sure exactly, but it seems some stuff like this may be useful:
a) Total time = Time taken to fly 1st bit + 10 sec + Time taken to fly 2nd bit
b) Total Displacement (or DISTANCE, if the acceleration wanted is not a vector, but it seems to be) = Displacement for 1st bit + Displacement for 2nd bit (=v1 + v2)