Use the formula: wavelength = speed ÷ frequency
Write it as: λ = v / f
For light in vacuum, use: λ = c / f
For sound or other waves, use the wave speed in that medium
Make sure speed is in meters per second and frequency is in hertz
The wavelength will be in meters
Example: if speed = 300,000,000 m/s and frequency = 100,000,000 Hz, then wavelength = 3 m