The first and only stop of the flight can be Boise, Omaha or Chicago. This means there are 3 possibilities where to stop. From thee the flight connects to New York City, either to La Guardia or to JFK Airport.
1 possible route: Seattle-Boise-La Guardia
2 possible route: Seattle-Omaha-La Guardia
3 possible route: Seattle-Chicago-La Guardia
4 possible route: Seattle-Boise-NYC
5 possible route: Seattle-Omaha-NYC
6 possible route: Seattle-Chicago-NYC
Or in other words: There are two airports where the flight can arrive and for each of them there are three possible routes. So, in total there are 2*3=6 possible routes.