it means you need to prompt the user for a radius

that is used to measured from the coordinate of the city

that they provide the zip code.

So if user input ZipCode as 12345 and radius 20miles, you need to display all airports that is within the range, in other words, covered by a circle drawn with the specific radius. 

Keep in mind, earth is not flat like the Professor said, research a bit on great circle distance and apply to your app