The first segment started at $(0,0)$ and stopped at $(7,3500)\text{.}$ This implies, Kato started at the starting point, traveled $7$ hours and reached a point $3500$ feet higher in elevation from the starting point. The slope of the line is $$\frac{\Delta y}{\Delta x}=\frac{3500}{7}=500$$ and with units, that is $500$ ft/hr. In context, Kato was running, gaining $500$ feet in elevation per hour. The third segment started from $(19,3500)$ and stopped at $(23,0)\text{.}$ This implies, Kato started this part of his trip from a spot $3500$ feet higher in elevation from the starting point, traveled for $4$ hours and returned to the starting elevation. The slope of the line is $$\frac{\Delta y}{\Delta x}=\frac{-3500}{4}=-875$$ and with units, that is $-875$ ft/hr. In context, Kato was running, dropping in elevation by $875$ feet per hour. What happened in the second segment, which started at $(7,3500)$ and ended at $(19,3500)\text{?}$ This implies he started this portion $3500$ feet higher in elevation from the starting point, and didn’t change elevation for $19$ hours. The slope of the line is $$\frac{\Delta y}{\Delta x}=\frac{0}{19}=0$$ and with units, that is $0$ ft/hr. In context, Kato was running, but neither gaining nor losing elevation.