You can put this solution on YOUR website! take a central point in the polygon and join it to all vertices.you will get as many triangles as there are sides to the polygon.sum of all the areas of all the triangles will give the area of polygon.triangle area can be found by normal standard formula.you need to know exactly about the polygon to work out specific case.
for example for regular hexagon,you will get 6 equal equilateral triangles..so area of regular hexagon=6*{a*a*sqrt3/4) where a is the side of hexagon
