Area of a circle diameter. Viewed sideways it has a base of 20m and a height of 14m. Visual on the figure below.
Then you can use the formula.
One method of deriving this formula which originated with Archimedes involves viewing the circle as the limit of a sequence of regular polygons. Visual on the figure below. Then we want to calculate the area of a part of a circle expressed by the central angle. A C2 4p.