This paper deals with the problem of finding solutions to the unknowns of Boolean equations, which was a major method of reasoning in 19th-century logic algebra, a problem that Schroeder studied as the Auflö sungsproblem (solution problem). It is closely related to the modern concept of Boolean unification, and is usually presented in an algebraic setting, but it can also be useful for knowledge representation based on predicate logic. This paper shows that this problem can be modeled based on first-order logic that extends two-quantization. Many previous results are applied, the basis of the algorithm is revealed, and the connection with two-quantization elimination and Craig interpolation is clarified. The set of solutions for first-order inputs is recursively enumerable, but developing a constructive method is still a difficult task. This paper presents several constructible cases based on Craig interpolation.