If you know the frequency, then the equation to find the wavelength
of ANY wave is
Wavelength = (wave speed) / (frequency) .
Since we're talking about light, we'll use 3 x 10^8 m/s for a speed.
Wavelength = (3 x 10^8 m/s) / (4 x 10^14 /sec) =
7.5 x 10^-7 meters =
750 nanometers