What is the frequency of a wave with a wavelength of 6.40 x 104 meters?

The frequency of the wave is 4.68 kHz.

The formula connecting frequency f and wavelength λ is

fλ = c,

where c = the speed of light [2.998 * 10^8 m·s^ (-1) ]

We can rearrange the formula to get

f = c/λ

∴ f = 2.998 * 10^8 m·s^ (-1) / 6.40 * 10^4 m = 4.68 * 10^3 s^ (-1) = 4.68 * 10^3 Hz

= 4.68 kHz