The coordinate choice is a convention. Reversing the positive axis reverses directional signs consistently without changing the physical event. A negative velocity means one direction, not automatically slowing down.

The model has a domain of validity. Explain its assumptions before using it in a changed context, then compare your prediction with evidence.

• Choose which direction is positive. Motion that way has a plus sign; motion the other way has a minus sign. Write the direction in words in your final vector answer.
• Speed is nonnegative magnitude; velocity carries direction. State your axis choice.