The position of an object at time t is given by s(t) = -9 - 5t. Find the instantaneous velocity at t = 4 by finding the derivative.

[tex]\bf s(t)=-9-5y\implies \left. \stackrel{v(t)}{\cfrac{ds}{dt}=-5} \right|_{t=4}\implies -5[/tex]

since the derivative is a constant, it doesn't quite matter what "t" may be.