A flare is shot up from the deck of a ship, the initial upward velocity of the flare is 30 m/s. The height of the flare is given by

The Question is:

Determine the instantaneous velocity of the flare at t=3 seconds by using the slopes of the secants.

In other examples I found the instantaneous velocity by creating a table and using numbers that get closer and closer to 3 seconds until I can reach a conclusion.....but how do I use the slopes of the secants to find it?

Thank you