Question

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Answer 1

If an object is slowing down, it's decelerating and is therefore acting against (negative) the direction of motion. If it is speeding up, it's accelerating in the positive direction of motion. However, you must be careful which way you define the positive and negative directions, they're arbitrary words.

Answer 2

Acceleration is a vector which means it has a magnitude and a direction. Positive and negative are just directions, I could say the acceleration due to gravity is -9.8ms/s or +9.8ms/s. I think it the context of your question if the object is slowing the acceleration is acting in the opposite direction with relation to its motion yes.