The position of an object at time t is given by s(t) = 3 - 4t. find the instantaneous velocity at t = 8 by finding the derivative.

For this case, we have that the equation of the position is given by:
 
 To find the velocity, we must derive the equation from the position.
 We have then:
 
 Then, we evaluate the derivative for time t = 8.
 We have then:
 
 
 the instantaneous velocity at t = 8 is:
 

- 4

Explanation: 

As the question tells, the instantaneous velocity is the first derivative of the position.

1) position equation given: s(t) = 3 - 4t

2) derivative, v(t) = s'(t)

s'(t) = [ 3 - 4t]' = (3)' - (4t)' = 0 - 4(t') = - 4

3) Then, the velocity is constant (does not depends on t), and its value is - 4.