How to achieve a parametric position of a rectangle?

Hi all,

I am looking for a way to have a parametric position of a rectangle. I define two global variables: x_center and y_center.

Now I draw a center-rectangle.

I would like to have that the position in the space of my object changes when I change the two global variables. So there should be a way to assign x_center and y_center to the coordinates of the center of the rectangle, but I can't find it.

Can anybody help me?

Thank you in advance!

Federico