A question from my text book states:
Would the answer be -5,000 m/s^2?
Yep, that works. However, since the coordinate system wasn't defined for you, why not define the positive direction to be the direction of the acceleration? If you define the initial velocity to be -20 m/s and final velocity to be 30 m/s, the acceleration is a positive 5000 m/s2.
it's a good idea not to use the word "deceleration" when you are working out these things. Use the idea of vectors - which have signs.
If you draw a big arrow on your (imaginary, perhaps) diagram to define the direction you will use for Positive, stick to it, and, if the acceleration comes out as negative then this means the velocity in that direction is decreasing. If you want to talk to 'Joe Public', at that point, you can then use the word "deceleration'.
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