Answer: -3
Work Shown:
s(t) = -9 - 3t
s ' (t) = -3 ... apply the derivative
The position function is linear, so the derivative is constant. The slope of any straight line is the same regardless of where you are on it, so that's why the velocity is a single fixed number. Put another way, the slope of the line is exactly the velocity (since slope = rise/run = change in distance over change in time). It doesn't matter what the value of t is, the velocity will be -3