Can i get some detailed explanation for this question?

. and County C. These countries m and \"Pill? mions for product x; _ 55-2P553=l5l}*-Pg;S(-=4F~.Dr=tinPr, where subscripts denote countries of the corresponding variables. Now consider a customs union between CountryI A and Country B under which these fill-\"1.33: of this impose a common external tariff on imports from Country C. Derive the ophmI Sweltltgoun' C customs union that maximizes the joint total surplus of Country A and B, assuming \"'5 does not react to such a common external tariff If no tariff and no expert subsidy]. The optimal specic tariff of\" the customs union =