Code:

solve with coordinate geometry triangle area formula:

Spoiler Alert ...

let points A = (0,2), B = (2,0), C = (4,2), D = (2,4), P=(x,y)

triangle area = (ax(by - cy) + bx(cy - ay) + cx(ay - by)) / 2, points counter-clockwise

right bottom = 2 + area(A,B,P) = x + y

left bottom = 2 + area(B,C,P) = 4 - x + y

right top = 2 + area(A,P,D) = 4 + x - y

right top = 4*4 - (4 + x - y) - (x + y) - (4 - x - y) = 8 - x - y = 8 - right bottom

right top + left bottom = 8 = 16 cm^2 + 32 cm^2 = 48 cm^2

sx area = 8 - right bottom = 48 cm^2 - 20 cm^2 = 28 cm^2