This is a linear function so it is going to have a constant rate of change (i.e. constant velocity).
First differentiate:
s(t) = 3 - 4t
s'(t) = -4
s'(8) = -4
(The instantaneous velocity at any point will also be -4)
Insert units here if you have them. Like if the y axis says metres, and the x axis says seconds, then your answer should be -4m/s.
Hope this helps.
