Microwaves are electromagnetic waves, whose frequency is between 300 megahertz (MHz) and 300 gigahertz (GHz) and a wavelength in the range of 1 m to 1 mm; therefore the microwave range is included in radio frequency bands.
The speed of a wave is the relationship between the wavelength and the period, that is:
v = λ/T
v, is the wave velocity.
λ, is the wavelength.
T, is the wave period.
As the period is the inverse of the frequency (T = 1/f), then we can write the velocity as follow:
v = λ*f
In the case of electromagnetic waves, the speed (v) is equal to the speed of light in the vacuum (c = 3*10^8 m/s); so that by solving the above equation for the frequency, we have:
f = v/λ
f = (3*10^8)/(6*10^-2)
f = 0.5*10^10 = 5*10^9 Hz = 5 Ghz
So the frequency (f = v/λ) of a microwave of 6.0 cm in length is 5 Ghz.