For `t = 0` to `2`: `(Deltas)/(Deltat)=4/2=2\ text(ms)^-1`

For `t = 2` to `7`: `(Deltas)/(Deltat)=0/5=0\ text(ms)^-1`

For `t = 7` to `8`: `(Deltas)/(Deltat)=(-4)/2=-4\ text(ms)^-1`

(The velocity is negative, since the particle is going in the opposite direction.)