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The displacement of a particle at time 
after leaving a fixed point O is 
. The diagram shows a displacement-time graph which models the motion of the particle. The graph consists of 4 straight line segments. The particle travels 
in the first 
, then travels at 
for a period of . The particle then comes to rest for a period of 
, before returning to its starting point when 
. Find the velocity of the particle during the last of its motion.
