In this article the correctness of a linear inverse problem with seminonlocal boundary conditions for a three-dimensional equation in a parallelepiped is considered. The equation itself is a fourth order mixed type equation of the second kind. The existence and uniqueness theorems for a generalized solution of the inverse problem in a certain class of integrable functions are proved using the methods of Fourier, "εregularization", a priori estimates, approximating sequences and contracting mappings.