Floor rotation in relation to x axis

Hello guys,

I want to rotate rebar in order to fit inside a certain floor. The problem is that the rebar definition is set to the coordinates 0,0,0 and they go along the X and Y axis, and if I have a rotated floor they still go along those lines, as seen in the image below.

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My idea to solve the problem is to get the rotation of the floor in relation to the X axis by finding the biggest side of the floor and measure it’s rotation to X axis. So far i can find the biggest side but i cant read it’s rotation.

Thank you in advance.