The absolute abundance of potassium has been determined for the first time from X-ray solar flare line and continuum spectra. The absolute and relative abundances of Ar and S have also been determined. Assuming that the flare plasma is coronal, and since potassium has the lowest first ionization potential (FIP) of any common element on the Sun, this determination is of importance in the continuing debate concerning the nature of the coronal/photospheric element abundance ratios, which are widely considered to depend on the FIP. The measurements were made with the RESIK crystal spectrometer on the Coronas-F spacecraft. A differential emission measure DEM~exp(-βTe) was found to be the most consistent with the data of three models considered. We find that the K/H abundance ratio is (3.7+/-1.0)×10-7, a factor of 3 times photospheric. Our measured values of the Ar/H ratio, (2.8+/-0.2)×10-6, and of the S/H ratio, (2.2+/-0.4)×10-5, are equal to previous coronal and photospheric determinations to within uncertainties. These measurements therefore fit a pattern in which low-FIP elements are enriched in the corona by a factor 3 and in which high-FIP elements (including S) have equal coronal and photospheric abundances.