Vector addition and motion diagram

[arukia]

Mastering physics Knight. 2nd edition.
A figure shows two dots of a motion diagram and vector $\vec{V}_{1}$ Copy this figure and add vector $\vec{V}_{2}$ and dot 3 if the accelration vector $\vec{a}$ at dot 2 (a) points up and (b) points down.

$\vec{V}_{1}$ is a vertical vector pointing upward with two points, labelled 1 and 2. With 2 being the topmost point. The vector is 15 mm in length.

The answer in the back of the book shows:
a) Two vectors with a total of three points. They point up and a acceleration vector next to the second point which points down. The topmost vector is slightly shorter than the bottom one.
b) Shows the same thing.

I don't understand what's being asked or what there doing here.

 

[Kavik]

Are the dots supposed to represent position? I would guess that's the case for a motion diagram. It's been a while so I'm not 100% sure, though.But if that's the case, what can you say about the acceleration based off of what the position is at each dot (I also think with a motion diagram, each dot is separated by an equal amount of time, e.g. 1 dot every 1 second or something - again I'm not 100% sure, though. But that's needed in order to say something about velocity - which I'd assume is the vectors - well, average velocity)

Hopefully someone else can chime in that is more confident.