Average Velocity

2) The displacement (in feet) of a certain particle moving in a straight line is given by s = (t^3)/6, where t is measured in seconds. Find the average velocity over the time period [1, 3].

Where do I plug 1 and 3? Do I plug 1 and 3 at all? Why is 1 and 3 inside the brackets? What does this mean?

NOTE: The textbook has the following equation for average velocity:

Average velocity = distance traveled/time elapsed

What can I do with this information?