What is the formula for finding the area of a regular polygon with perimeter p and apothem length a

If you know the perimeter you don't need the apothem because:

A(n,s)=ns^2/(4tan(180/n)) where n=number of sides and s=side length and A is the area of any regular polygon

When you know P you know s as P/n which would make the above formula:

A(n,P)=P^2/(4ntan(180/n))