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Given is period of an eclipsing binary star system is 34 days, rest wavelength is 6563 angstroms while max doppler shift of 2.34 angstroms

I used the doppler equation to figure out the orbital speed of the star; now the question says to use the simplistic model of being in a circular orbit about the other star , calculate the sum of the masses of the stars in solar units.

I know I have to use the modified kepler 3rd law to find the combined mass but how do i get 'a' or semi major axis? Do I just multiply the orbital speed by the period in seconds to get this?

thanks