The following notes will help you understand position, velocity, and acceleration are positive or negative:

Position:
+ when R of origin
- when L of origin

Velocity:
+ when moving R
- when moving L

Acceleration: (Think number line: the change from Vi to Vf tells you the sign of the acceleration)
+ when speeding up in + direction
or slowing down in - direction
- when slowing down in + direction
or speeding up in - direction