Kinematics of motion in a straight line — practice question

The diagram gives the velocity-time graph for a bus’s motion. The bus begins from rest and accelerates uniformly for $8$ seconds until it reaches $12.6\text{ m s}^{-1}$. It then continues at this speed for $40$ seconds. After that, it slows down uniformly in two stages. From $48$ to $62$ seconds the bus decelerates at $a\text{ m s}^{-2}$, and from $62$ to $70$ seconds it decelerates at $2a\text{ m s}^{-2}$ until it stops.