So the initial velocity is 15 m/s, the final velocity is 0 since it's at a complete stop and time is 10 seconds. Therefore:

Therefore, the acceleration is -1.5 m/s^2. The reason it's negative is due to the fact that the vector is going against it's original movement since it's decelerating.
Answer:
100years later
Explanation:
Because the lights will arrive at world after 100 years later.

where
is the gravitational constant and is about 

:D