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))