let r= the radius of the circle

A = r^2*pi the area of a circle

r^2*pi = 81*pi divide both sides by pi

r^2 = 81

r^2 = 9^2 and r>0

r = 9 ft

C = 2*r*pi the circumference

C = 2*9*pi

C = 18 pi ft

the circumference is 18 pi feet.