Question

What is centre of mass of triangle.

Answer 1

If a triangle has vertices with coordinates:

$$A = (x_A,y_A)\\ B = (x_B,y_B)\\ C = (x_C,y_C)$$

then the center of mass has the following coordinates:

$$O = \left( \dfrac13(x_A+x_B+x_C), \dfrac13(y_A+y_B+y_C \right)$$

See the derivation and examples here: https://www.math-only-math.com/centroid-of-a-triangle.html