How do I calculate this?

Below is an "oracle" function. An oracle function is a function presented interactively. When you type in a t value, and press the f> button the value t) appears in the right hand window. There are three lines, so you can calculate three different values of the function at one time. The function t) represents the height in feet of a ball thrown into the air, t seconds after it has been thrown. Calculate the instantaneous velocity 0.31 seconds after the ball has been thrown. Instantaneous velocity at 0.31 : [j t no 03099999 --f-> 0.6324002919998253 0.31 f> 0.6328999999999853 0.32 ---f--> 0.6015999999999853