Final answer:
If you're a 3D shape with 6 faces and 12 edges but not a cube, you are a rectangular prism, which has faces in the shape of rectangles and can have different edge lengths.
Step-by-step explanation:
If you are a 3D shape with 6 faces and 12 edges, and you are not a cube, then you are most likely an octahedron. An octahedron is a polyhedron with eight faces, however, when the question describes the shape as having six faces and twelve edges, it seems to be describing a rectangular prism.
An octahedron usually has eight faces and six vertices, resembling a double pyramid. However, a rectangular prism, which is also known as a rectangular cuboid or right rectangular prism, fits the described shape perfectly, as it has 6 faces and 12 edges, with each face being a rectangle. Unlike a cube, the faces of a rectangular prism are not necessarily squares, which means its edges can be of different lengths.