Let's enumerate the number of times the plane is on the ground (that is, the graph touches the horizontal axis):
1 - At the start of the trip (Departure from Chicago)
2 - First stop on the way to New York
3 - Second stop on the way to New York
4 - Third stop on the way to New York
5 - Arrival in New York
6 - First stop on the way back to Chicago
7 - Arrival in Chicago.
So we have a total of 7 times when the graph touches the horizontal axis.