You don't need the apothem if the side length is known, the area can be expressed as:
A(n,s)=ns^2/(4tan(180/n)), n=number of sides and s=side length so
A(7,28)=(7*28^2)/(4tan(180/7))
A≈2848.987 ft^2
A≈2849 ft^2 (to the nearest tenth of a square foot)
