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.