When Does Change in Velocity Use v=v2+v1 or v=v2-v1?

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fakecrash2011
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in this question change v=v2-v1: a car traveling at 15m/s [N] executes a gradual turn, so that it then moves at 18m/s[E]. Calcue change in veloctiy.

but in this question change v= v2+v1: A plane's velocity changes from 200km/h[N] to 300 km/h [N 30 W]. Find the plane's change on velocity.

How do i know when to use which formula?
 

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fakecrash2011 said:
in this question change v=v2-v1: a car traveling at 15m/s [N] executes a gradual turn, so that it then moves at 18m/s[E]. Calculate change in velocity.

but in this question change v= v2+v1: A plane's velocity changes from 200km/h[N] to 300 km/h [N 30 W]. Find the plane's change on velocity.

How do i know when to use which formula?
Change in velocity is Δv = v2 - v1 . Never is v2 + v1
 
what i mean is how do you know when to add or subtract vectors?