If a problem says the acceleration is some positive value than solve using that value, a negative acceleration is said to be deceleration. E.g. a car decelerating at 10 m/sec can be said to be accelerating at -10 m/sec.
If a problem states decelerates at A, then use -A for acceleration in the classic equations which are for acceleration. If a problem says accelerates at a negative value like -A the use -A as the value for acceleration, it can also be said to be decelerating at A.
