这道题，用大白话讲解---Here is the extracted content from the image: **Problem 21:** 如图，正方形 OABC 的顶点 A、C 分别在 x、y 的正半轴上，点 B 的坐标为 (4, 4)，一次函数 y = -1/2 x + b 的图象与边 OC、AB 分别交于点 D、E，并且满足 BE/AE = 3. 点 M 是线段 DE 上的一个动点。 (1) 连接 BD、OE，求证：四边形 ODBE 是平行四边形； (2) 作 BP ⊥ DE 交 OA 于 P，当 △OMP 面积为 2.6 时，求 M 点的坐标； (3) 设点 N 是 x 轴上方平面内的一点，以 O、D、M、N 为顶点的四边形是菱形，求点 N 的坐标. **Chart Description:** Type: Coordinate geometry diagram. Main Elements: - Coordinate Axes: X-axis and Y-axis intersecting at the origin O. Positive directions are indicated by arrows. The positive x-axis and positive y-axis are shown. - Square OABC: Vertices O, A, B, C are labeled. O is at the origin. A is on the positive x-axis. C is on the positive y-axis. B is in the first quadrant. The sides OA and OC are aligned with the positive axes. AB is a vertical line segment, and BC is a horizontal line segment. - Line: A straight line is shown passing through the square. It intersects the y-axis segment OC at point D and the vertical side AB at point E. - Points: O, A, B, C, D, E are labeled. D is on OC. E is on AB. - Labels: O, A, B, C, D, E, x, y. - Relative Position: O is at the origin. A is on the x-axis, C is on the y-axis, B is above and to the right of A and C respectively. The line intersects OC (y-axis) and AB (a vertical line). Note: There are two identical diagrams shown side-by-side. The description above applies to both.