The velocity function is given by the derivative of the displacement function.
[tex]v(t)=\dfrac{\mathrm ds(t)}{\mathrm dt}=-\dfrac2{t^3}[/tex]
You want to find the velocity at [tex]t=2.5[/tex], which would be
[tex]v(2.5)=-\dfrac2{2.5^3}=-\dfrac{16}{125}=-0.128[/tex]
