In our case, is a triangle with vertices , and . From this, we can come up with the equation of the line that connects the points and and the points and . I leave it for you to verify that the equations of the lines are and respectively.
Thus, it follows that if , then
Now, to find center of mass, we need to evaluate and .
From there, it follows that
Note that and .
I leave it for you to simplify the two integrals and figure out the coordinates for the center of mass.
Can you take it from here?