Let be how many miles apart the cities are
Let be the cost of the trip.
then for the first trip, we have
and for the second trip, we have
Now let and
then the slope of the line connecting these two points is given by:
now, by the point slope form, we have the equation of the line given by:
plug in the values when you find and solve for . after that, you have the relationship. to find how much a trip between cities 1000 miles apart costs, just plug in into the equation of the line and solve for