It's better to understand how to find the area of regular polygons: 1/2 * a * p, where a stands for the apothem and p stands for the perimeter of the polygon...
Mark the center and create a triangle joining two of the vertices. From the center of the polygon, drop a perpendicular line(this is the apothem !)
The central angle is 360 / number of sides, so 360/12=30.
Since the perpendicular line cuts the angle of 30 degrees in half, we have two fifteen(15) degree angles..
By the Law of Sines, we have sin154.5=sin75y. Solving for y, gives the apothem, which is approximately 16.794 centimeters.
The perimeter of the polygon is 9 * 12 =108 centimeters.
Thus, the answer is 12∗16.794∗108≈906.9 centimeters squared.