The relationship between the frequency and the wavelength of light is   C = l f.  Where C is the speed of light in meters per second, l is the wavelength of the color of the light in meters, and f is the frequency of that color of light in Hertz.

Thus the equation for l is: l = C / f   Likewise the equation for f is: f = C / l .

The l of the first subharmonic of the Upper Sideband of the mixture is lMix = C / (f1 + f2 ) / 2

Where f1 is the first frequency of the mixture, and f2 is the second frequency of the mixture.

lMix = C / (C / l1 + C / l2 ) / 2 by substitution of the frequency equations for the two frequencies.

lMix = 2C / (C / l1 + C / l2 ) by simplification.

lMix = 2C / C (1 / l1 + 1 / l2 ) by factoring C out of the bracketed term.

lMix = 2 / (1 / l1 + 1 / l2 ) by cancelling out the C's in the numerator and denominator.