A motorcyle goes from 30kmhr to 9kmhr in 2 s what is its acceleration?

The motorcycle averages a negative 10.5 kmph acceleration. That is, the bike decelerates from 30 kmph to 9 kmph in 2 seconds. That's a change of 21 kmph over a period of 2 seconds, and that change is negative. The 21 kmph over a period of 2 seconds represents a 10.5 kmph change every second. Note that acceleration is a vector quantity. It has both magnitude and direction. We "fudged" here and assigned a "direction" based on the direction is it traveling. It's not a biggie, but do not make the mistake of leaving off a vector when considering a velocity (which is speed plus a direction vector) or acceleration, like in this problem.