Kinematics of motion in a straight line — practice question

At time t seconds after the particle has left a fixed point O, its velocity is v\,\text{m s}^{-1}. The diagram is a velocity-time graph representing the particle's motion from t = 0 up to t = T. It is formed from four straight-line sections. From rest, the particle increases its speed to V\,\text{m s}^{-1} in 4 s, and then slows down at 3\,\text{m s}^{-2} until it is momentarily at rest after 6 s. The particle then moves back towards O, reaches a greatest speed of 3\,\text{m s}^{-1}, and finally comes to rest again when t = T.