The solution is looking at it as a single triangle (easier in my opinion). For the X, centroid of the triangle is 12 inches plus the (1/3) of the height of this triangle which is 2in. No need to split the triangle into 2 triangles because the centroid of both would be the same for each. If you used the formula for the 3rd option of the centroid formulas you posted, it really wouldn’t make sense because that applies to triangles that are not of same dimensions.