In order to derive the base of a triangle from its area, you need its height as well.
In fact, if we solve the area formula for the base, we have
So, the base length would be
where h is the height relative to the base you're interested in.
First isolate the group with h in it
minus 2pir^2 from both sides
s-2pir^2=2pirh
divide both sides by 2pir
(s-2pir^2)/(2pir)=h