An object is traveling at a constant speed of 12.0 m/s when it slows to a stop. if it takes 10.0 m for the object to stop, what is the magnitude of its acceleration?

Magnitude of acceleration =

         (change in speed) divided by (time for the change).

Change in speed =

           (speed at the end of the time) - (speed at the beginning of the time)

       =      (zero)            minus      (12 m/s)  =  -12 m/s

Time to stop:
I'm not sure whether you meant to write 10 seconds or 10 minutes.

If it's 10 seconds, then

       Acceleration  =  (-12 m/s)  /  (10 seconds)  =  -1.2 m/s²

If it's 10 minutes (600 seconds), then

       Acceleration  =  (-12 m/s)  /  (600 seconds)

                           =     - 0.02 m/s²  .