Doppler Spectroscopy - Finding mass of an exoplanet
To use the Doppler shift, you need to know by how much the light is blue/red-shifted. 

Suppose you have a light curve where the wavelength varies between $\lambda_1$ and $\lambda_2$. Using these wavelengths, the Doppler shift will give you an interval of radial velocities. This gives you a value for K that you can plug into your last equation to derive $M_p$.