Answer:
a = -6/t (on the opposite direction of the bicycle motion)
Step-by-step explanation:
For all problems of physics we need to set the positive axis direction. and the origin. In our case the origin location does not make difference since we are only interested in velocities and acceleration. So I'm gonna set the positive axis in the same direction that the bicycle is moving. Therefore, our initial (10 m/s) and final (4 m/s) velocities are also positive.
However we have some problems. We do not know how this bicycle was decelerated (constant or not). Besides we do not have the information like time or displacement of this deceleration.
So, supposing an uniform deceleration and using the velocity time function for uniform "a", we get:
v = vo + at (set the initial time to 0s)
4 = 10 + at
a = -6/t
So you only can determine the acceleration if you have either the time or the displacement (using Torricelli equation). But we know that our bicycle acceleration points against the direction of the bicycle motion due to the negative sign.