The points \( (2, 5) \) and \( (5, 1) \) are the two opposite vertices of a rectangle. If the other two vertices are points on the straight line \( y = 2x + k \), then the value of \( k \) is:
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For any parallelogram (including rectangles and squares), the center of the figure is the midpoint of the diagonals. If a line contains one diagonal, it must contain this center point.