It is given that the scale used in the map is,

For 9 inches,

Thus, a distance of 9 inches on the map corresponds to 150 miles in actual.
For 15 inches,

Thus, a distance of 15 inches on the map corresponds to 250 miles in actual.
For 21 inches,

Thus, a distance of 21 inches on the map corresponds to 350 miles in actual.