In an analysis of the following reaction at a certain temperature, Br2(g) + Cl2(g)<-->2BrCl(g) the equilibrium concentrations were found to be [Br2]=3.9x10^-3 M, [Cl2]=2.6x10^-2 M, and [BrCl]=1.5x10^-2 M. Write the equilibrium expression, and calculate the equilibrium constant for this reaction at this temperature.