On a boundary value problem for a third-order equation of parabolic-hyperbolic type in a triangular domain with three lines of change of the equation type

We consider one boundary value problem for a parabolic-hyperbolic type equation in a triangular domain with three lines of type change, when the characteristic of the first-order operator is parallel to the axis $y$. The theorem of existence and uniqueness of the solution of the formulated problem is proved.