Unless the area is a regular polygon (or a circle) you cannot.
More information is required in order to answer this question. What is the shape for which the perimeter is given? If it anything other than a circle or regular polygon it is not possible to determine the area. For a circle or regular polygon, the answer will depend on the shape.
Area of regular polygon: 0.5*apothem*perimeter
Perimeter = 2*Area/Apothem.
Find the apothem of a regular polygon with an area of 625 m2 and a perimeter of 100 m.
The answer will depend on the number of sides in the polygon. Since that information is not given, there can be no answer.
Area of regular polygon: 0.5*20*3 = 30 square feet
A = (1/2)Pa A being the area, P being the perimeter of the regular polygon, and the apothem length being a.
Such a polygon is not possible.
The formula used to find the area of any regular polygon is A = 1/2 a P where the lower case a stands for the length of the apothem and the uppercase P stands for the perimeter of the polygon.
The area of a polygon is not enough to determine its shape - unless it is a regular polygon. There is nothing in the question to suggest that the octagon is regular.
Area of regular polygon: 0.5*3*20 = 30 square feet