For equilibrium equations you write K = [products]/[reactants] and in this case the chemical equation for the dissolution of lead (II) bromide is PbBr2(s) --> Pb2+(aq) + 2Br-(aq) ; since only aqueous products/reactants are included in equilibrium expressions (hence the brackets for concentrations), the solid PbBr2 is not included, only the two products are. Therefore the expression should simply read K = [Pb2+][Br-]2
The Br- is squared in the equation because coefficients in the chemical equations become exponents in the equilibrium expressions. See bolded PbBr2(s) --> Pb2+(aq) + 2Br-(aq) K = [Pb2+][Br-]2
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