Displacement-time graph problem 1

The displacement of a particle moving in a straight line is metres at time seconds after leaving a fixed point O. The particle starts from rest and passes through points P, Q and R, at times , and respectively, and returns to O at time . The distances OP, OQ and OR are , and respectively. The diagram shows a displacement-time graph which models the motion of the particle from to . The graph consists of two curved segments AB and CD and two straight line segments BC and DE. Find the speed of the particle between and .