You can abuse the fact that PI is Circumference/diameter.
This means that, to find the diameter, you would have to do the following:
PI = circumference/diameter
(You can choose the number of decimals of PI, based on how accurate you want the diameter. 3.14 tends to work for most cases.)
After youve done this, you would be able to find the diameter like this:
PI*diameter = circumference
diameter = Circumference/pi
Then, you can use the diameter/2 to find the radius, like this:
radius = diameter/2
After this, you can simply use the formula for the surface area of a circle, like this:
SA = PI*Radius^2
I might be able to add an example, if really needed.
