[Physics II] Instantaneous velocity

Alright, so the problem goes as follows:

A cyclist travels at a velocity of 20 m/s in a horizontal surface, he starts braking and stops in 10s. Calculate the velocity at t=8s

After some time searching I have come across many examples that approach the problem in different ways that don't resemble what I've seen in class. From what I understand, what I have to do is get the gradient of the formula and then plug in time, which is 8. But I can't seem to find how to get that formula. Any help would be appreciated, thanks!