A = (7, 1)
B: (9, 6)
pts: {(0, y(A)), A, (x(A), y(B)), B}
pts' = Mirror(pts, xAxis)
poly1 = Polygon(Join[{pts, Reverse[pts']}])
TR: B + (1, 1)
BL: (-1, (-y(TR)))
C = Centroid(poly1)