I think this is perhaps best explained the long way...
The pressure on the window obviously varies from top to bottom, less at the top more at the bottom.
Consider the window in small horizontal strips.
You can readily work out the pressure on each strip.
Start 2 sums adding the pressures together, one starting from the top and the other starting from the bottom.
The higher pressures at the bottom will cause the sum starting at the bottom to increase more rapidly than the sum starting from the top.
Where the two sums reach the same value (somewhere closer to the bottom than the top) is the centre of pressure point you are after.
As you consider this and look at the geometry you will probably realise there is a shorter way to find the answer purely based on the geometry.
However jumping straight to the shortcut would perhaps mask the fuller meaning of the process.
Also this longhand way can be extended to much more complex shapes, which might not be so amenable to shortcuts.
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