Notice what each axis represents.
negative acceleration = decreasing velocity. The first time this happens is between t = 2 and t = 4. When else does this happen?
On what interval is Tiffani's velocity increasing the most? Remember, this is a velocity graph.
0 acceleration means that velocity is not changing. It does NOT mean that velocity is 0. What does it mean then? Notice that Tiffani was moving with 0 acceleration during three intervals on her journey.
Displacement can be determined by calculating the area under a velocity curve.