From the graph it is evident that the acceleration is decreasing with time.

Also, \(a \propto-t\)

\(\Rightarrow a=-k t \quad\) (decreasing with time)

To find velocity,

\(\frac{d v}{d t}=-k t\)

\(\int d v=\int-k t d t\)

\(v \propto-t^2\) or graph of velocity should be parabolic with a decreasing slope.