A dodecagon is a polygon with 12 sides, In a regular dodecagon the 12 sides have equal length and the polygon's area is the sum of the areas of 12 isosceles triangles with an apex of 360/12=30 degrees.
The radius 20cm forms the two equal sides of each triangle so the area is ½(202)sin30°=100cm2, and the area of the dodecagon is 1200cm2. (Compare this with the area of the circle with the same radius=400π=1257cm2 approx.)