如图，已知正方形 ABCD的边长为4，点E、F分别在边AD、BC上，将正方形沿着 EF翻折，点 B恰好落在 CD边上的点B'处，若四边形ABFE的面积为6，则线段 DE的长为？---**Diagram Description:** * **Type:** Geometric figure (Square with internal points and line segments). * **Main Elements:** * A square labeled ABCD. * The side AB is labeled with the length '4'. (Assuming ABCD is a square, all sides have length 4). * Point E is located on the side AD. The segment AE is labeled with the length 'x'. * Point F is located on the side BC. The segment BF is labeled with the length '3-x', and the segment FC is labeled with the length '1'. * Points A' and B' are located on the side CD. The segment B'A' is labeled with the length 'x', and the segment FB' (connecting point F to point B') is labeled with the length '3-x'. * Line segments are drawn connecting: A to E, E to F, F to B', and B' to A'. These segments form a polygonal chain A-E-F-B'-A'. **Extracted Text/Labels:** * Label on side AB: '4' * Label on segment AE: 'x' * Label on segment BF: '3-x' * Label on segment FC: '1' * Label on segment FB': '3-x' * Label on segment B'A': 'x' * Vertices: A, B, C, D, E, F, A', B'. **Note:** The image contains only a diagram with labels and no explicit question or options. The sum of the lengths on side BC (BF + FC = (3-x) + 1 = 4-x) would equal the side length 4 if ABCD is a square with side 4, which implies 4-x=4, leading to x=0. This apparent contradiction between the labels and the visual representation of x being non-zero suggests the diagram might be illustrative or part of a problem where 'x' is a variable or parameter.