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... She took a sheet of Origami paper and ...
Only folded the diagonal.
Then she took the rope and laid it down against the front of the building starting below the window and to the right as far as it would go. She stepped back about 3 or 4 times the length of the rope.
She took the paper triangle and kept it in front of her eyes. The surface parallel to the front of the building. And so also parallel to the rope and the line from the window down to the left end of the rope. By changing the distance from the paper to her eyes she could match the top and the bottom-left corners of the triangle with bottom of the window and the left end of the rope.
The right end of the rope was visible beyond the right corner of the triangle and so it was long enough.
In fact it is not neccesary to fold the diagonal but it makes a stiffer triangle.