Does a negative acceleration mean necessarily that something is slowing? explain

Yes, because imagine a car, as it races it has a positive acceleration. So imagine the car going faster, and at the point it is at and imagine it slowing down. Compared to its previous speed is it a positive or negative acceleration (after it slowed down)?
Negative.
this is because its speed is decreasing. When anything is decreasing it is negative.
In other words its speed is decreaseing meaning its having a negative acceleration

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