Divinding the polygon into right triangles, we have (20 x 2)= 40 right triangles in the polygon
Area of the polygon = 40 x Area of 1 right triangle inside
From the triangle above,
Area of the triangle = 1/2 x base x height
=1/2 x 5 x 31.57= 78.92 m^2
Area of the polygon = 40 x 78.92 m^2
=3156.8 m^2