Center of Mass - Matrix Representation

[akmon]

I´ve tried a little example to manage the program, but I didn´t get the results I expected. I want to know the center of gravity of a triangle. The coordinates are (0,0), (5,0) and (5,5). I suppose mass 1 for every points, so I write this matrix:

[[0 0 1][5 0 1][5 5 1]]

The gravity center sould be (10/3 5/3), but I don´t get that result. What am I doing bad? Thank you.