Theory:

A polygon is a closed shape that has at least three sides. Triangles, quadrilaterals, rectangles and squares are all types of polygons.
A polygon can be of any shape and can have any number of sides. There is no specific formula for calculating the area of a polygon. The best way is to split the polygon into known shapes whose area can be calculated individually.
Example:
Capture.PNG
 
In this figure, the area of polygon \(ABCDE\) is spilt in the form of two diagonals, \(AC\) and \(AD\). It divides the polygon into three parts.
 
Thus, the area of polygon \(ABCDE =\) Area of \(△ ABC\) \(+\) Area of \(△ ACD\) \(+\) Area of \(△ AED\).