In the formula C=λf, c dosen't change in the formula. What happens to f if λ increases? What happens to λ if f increases?
Here is the intro. Radio waves are also examples of electromagnetic waves. when you listen to a radio stationm the frequency that you find on the dial describes the frequency of the waves sent out by that particular station, since all electromagnetic waves move at the speed of light, you can calculate the wave length for a station.