From Mechanics

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

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

\displaystyle \frac{\left( 9-7 \right)\text{ m}{{\text{s}}^{\text{-1}}}}{\left( 0-2 \right)\text{ s}}=-1 \text{ m}{{\text{s}}^{\text{-2}}}