[Geometry] Equation for point of gravity

I have 2d object with more than 3 corners. The object is not correct by its sides.
I did find out some of equation for this but its look like im on the start and i can't go back or forward.
I did get the center of object with averige of all corners and i did separated object on the number of corners - 1 triangles. I did get out the point of gravity for every triangle in 2d object. But im stuck here i can find out.

For eny help i will be thankful.

I'm not familiar with this, so I may be wrong, but I believe the center of gravity for the object is the average of the centers of gravity for each of the triangles. (This from a brief look at Wikipedia)

Show us what you have already found. If you can show us the object, the solution may be easier to explain.