The small square on top will be in the upper half of the divided figure. Now consider the figure formed by moving this square to become an extra square on the left of the second row, as shown.
It may now be seen from the rotational symmetry of the figure that the line $PX$ splits the new figure in half- with that small square in the upper half.
Therefore the line which halves the area of the original figure cuts the edge $XY$ at $X$.