You must start with the definition of the equilibrium constant (Kc). Kc=[Concentration of Products]/[Concentration Reactants]. The concentration of the individual products or reactants must also be raised to the power of the stoichiometric coefficient. This means that the equation for Kc for htis problem should look like this:

Kc=[Br2]*[Cl2]/([BrCl]^2)

The values for Kc, [Br2], and [Cl2] are known. Using algebra you can rearrange this equation to solve for [BrCl]. That equation looks like this: