It is at the intersection of the hexagon's lines of symmetry, i.e. the middle! It is the midpoint of any diameter.
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Put one angle of each triangle at the center of the hexagon.
Each body has its own centre of gravity. The centre of gravity of two regular shapes - an equilateral triangle and a square will be different so why should the cog of a regular and an irregular shape not be different?
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"Having the same center" is the meaning of concentric. A regular hexagon in a concentric circle can be viewed by using the link and looking at the "active construction" window where there is a short animation of the construction of a hexagon inside a circle. It is posted by our friends at Wikipedia, where knowledge is free.
Assuming you have a regular hexagon, drawing a line between every other vertex and the center will give you three congruent quadrilaterals (that also happen to be parallelograms... in fact, they are rhombuses).