Maths behind rotating rectangle around start point instead of default center point

Hi

The following code rotates a rectangle around is start point instead of its center point

I don't understand the maths behind it and would like to know, does anyone know?

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The code looks incorrect/incomplete.

sorry its part of a much bigger part of code and this is the only part I don't understand, when i was deleting the other stuff I may have made the code incorrect. should be right now

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