Kinematics of motion in a straight line — practice question

The diagram presents the displacement-time graph for a particle’s motion. It begins from rest at point O and moves with constant acceleration of $a\text{ m s}^{-2}$, taking $4\text{ s}$ to cover a distance of $x\text{ m}$. It then goes back to O at constant speed of $40\text{ m s}^{-1}$, which lasts $2\text{ s}$.