a = 0 (slider from -3 to 3, step 0.01)
A: (a, 0.0)
B: (-(3.0), 3.0 - x(A))
C: (x(A), y(B))
D = (-3, 0)
poly1, a_1, c, b, d = Polygon(A, C, B, D)
area: (3.0 + a) * (3.0 - a)
Area: (x(A), area)