Question

A motor cyclist travelling at 12m/s decelerate at3m/s(squared) how far does he travel while coming to rest?

Answer 1

Use V=Vo+at to find time. Vo is the original velocity and V will be 0 because you are trying to see how far the motor cyclist has gone when he stops.
Find time, then use x=Xo+Vot+(1/2)at^2 to find distance.

Answer 2

Yes, Emily's approach will give the correct answer.

You can also use another kinematics relation to solve for distance x:
V1^2-V0^2=2ax
where
V1=final velocity = 0
V0=initial velocity = 12 m/s
a=acceleration = -3 m/s²
x=distance travelled in m.