A runner traveling with an initial velocity of 4.0 m/s accelerates at a rate of 6.0 m/s^2 for a time of 3.0 seconds What is the velocity at the end of this time?

This problem can be solved using the 1D dynamics equation, V = Vo + at, where Vo is the initial velocity, a is the acceleration, t is the time, and V is the final velocity at the end of that time. We have Vo = 4m/s, a=6m/s^2, t=3s, so you simply solve for V.