Question 161609
Circumference of any circle is:
C = (pi)/d
where
C is circumference
d is diameter
.
Area of any circle:
A = (pi)r^2
where
A is area
r is radius
.
Of course, radius is half the diameter or
r = d/2
.
Pulling it all together:
Since they gave you circumference, use the circumference formula to find d (diameter):
C = (pi)/d
18(pi) = (pi)/d
18=d
.
Extract radius (r):
r = d/2
r = 18/2
r = 9 cm
.
plug it into the formula for area of a circle:
A = (pi)r^2
A = (pi)9^2
A = 81(pi) square cm  (this is what they're looking for)