What is the frequency of a blue light with a wave length of 469 nanometers?

Frequency is in cycles per unit time

Wavelength is in length per cycle

To convert between the two you need the speed of the wave - which for light is the speed of light - which is given in length per unit time

Frequency = Speed of light/Wavelength

For a wavelength of 469 nanometers

Frequency is 639 Terahertz (THz) or 639,000 Gigahertz (GHz) ... rounded to 3 significant digits since the wavelength is only given to 3 significant digits