From Mechanics

Acceleration at any point is given by the gradient in a velocity-time graph.

On the first stage of the motion the gradient is given by

\displaystyle \frac{24 \text{ m}{{\text{s}}^{\text{-1}}}}{40\text{ s}}=0 \textrm{.}6\text{ m}{{\text{s}}^{\text{-2}}}