Paul is a physicist, observing the movement of a charged particle. The particle is moving in a straight line along the y-axis of an x-y plane. Its initial velocity was 3 m/s, and its final velocity was 6 m/s. At the end of Paul's observation, the particle's displacement was 3.75 meters in the positive direction. How long was Paul observing the particle's motion?

5.6 seconds

2.5 seconds

4.2 seconds

1.2 seconds