For any wave, the product of the frequency (f) times the wavelength (L) is equal to the propagation velocity (v):
v = f*L
If you know the wavelength and velocity, the frequency is given by:
f = v/L
If you know the frequency and velocity, the wavelength is given by:
L = v/f
For light propagating in a vacuum, of course, v = c = 2.998 *10^8 m/s, but this same relationship holds for sound waves, waves on a string or slinky, seismic waves from an earthquake, etc.. Naturally, the speed of these waves are quite different from c.
Just make sure you use the same units for time and length in all the variables (i.e., if frequency is in hertz (1/sec) then the speed needs to be expressed in distance per second, and if the wavelength is in cm, then the speed needs to be in cm per unit time.)