You need to use [tex]v^{2}-u^{2}=2as[/tex] where u is the initial velocity, v the final, s is the displacement and a is the acceleration. From this you can find s and then find t.
Imagine two trains running together on parallel tracks. One slows down, stops for a while and then accelerates back to cruising speed. The other one is now a good way ahead. What you have to find is the time separation between these two trains as they pass through the next station without stopping.