Acceleration= -0.45m/s/s How long to complete stop if initial velocity is 15.0m/s?

Here is a trick we use for such things. Given meters/second and meters/second/second we need to find an answer in seconds.

m/s x m/s^2 = m^2/s^3 so that is not the way to do it.

(m/s)/(m/s^2) = s so that is our method. Now plug in the numbers.

15.0/0.45 = 33.333 seconds

Check: 33.333 s x 0.45 m/s^2 = 15.0 m/s