Vector problem

At a certain time, race car has only a northward accelearation with a magnitutde of a1 and at a later time delta-t, the same race car has only an acceleration a2, what is the magnitutde of its average "j" at this time?

j= delta a/delta t

I solved the q and got the correct answer, but when I looked at princeton's vector diagram, I was a little thrown off.
Its a simple q asking to find the resultant change in acceleration

I drew an arrow pointing north (+a1) and then another from the head of the arrow pointing north eastward (+a2) to find the resultant a

Now, the explanation has an arrow eastward (+a2) and another from the head of the eastward arrow pointing south (-a1) I understand that there is no real difference between my and their vector diagram, but I'm a little weak at vectors, so want to be sure that my diagram was correct.

Secondly, they said a2-a1(incorrect answer choice) will be correct if accelerations given were vector accelerations, it is not true for magnitude of the accelerations????

 

here is the pic..

 

hmmm true, but then again, the q had one vector pointing north and one east.

Im confused by the first part of your explanation, even though we know the direction and the magnitutde, we can't just subtract vectors, unless they are in the same direction...ie both north or both east, if one is north and the other is east, you'd have to use the pythagorean thm,correct?