You must mean the area of a REGULAR hexagon, 
heptagon, or octagon, for there is no formula 
in general for all poygons with n-sides.  
Regular polygons are those with all sides equal 
and all interior angles equal.

The area of a n-sided regular polygon is

 

where r = the distance between the center to a vertex.

If only s, the length of a side of the regular polygon is
known, then you first must calculate r from this:



For a regular hexagon, use n=6
For a regular septagon, use n=7
For a regular octagon, use n=8  

Edwin