As the time changes along the horizontal axis the result is the changing position of the train measured on the vertical axis. The key word here is “change”

1. Use the graph to find the change in distance between the $9{:}00$ and $11{:}00$ A.M..
   $f(3) - f(1) \approx$ miles.
2. What is the change in time between the $9{:}00$ and $11{:}00$ A.M.?
   The change in time is hour(s).
3. What is the average speed (rate of change) between $9{:}00$ and $11{:}00$ A.M.?
   The average speed is ? hours miles hours per mile miles per hour
4. Use function notation (as in part a) to write an expression that represents the change in distance between the $10{:}00$ A.M. and $1{:}00$ P.M..
   The change in distance can be expressed as