RESIK Bragg spectrometer aboard CORONAS-F solar observatory collected spectra of hundreds of solar flares in the unique spectral range between 3.3 Å and 6.1 Å. The instrument's absolute and relative calibrations are now finished which allows for the first time to interpret the observations in terms of the differential emission measure (DEM) distribution. With known DEM shape it is possible to derive and study in detail flaring plasma composition and energy balance. Here we report results of using an improved Maximum Likelihood iterative algorithm (Withbroe-Sylwester) for determination of the DEM distributions for 7 January 2003 flare at 23:30 UT. The study is based on the absolute flux values observed in a number of strong H- and He-like transitions of K xviii, Ar xviii, Ar xvii, S xvi, S xv, Si xiv and Si xiii ions. In this respect, we use line and continuum X-ray emissivities as provided by CHIANTI package from SolarSoft.