Question 871450
Find the radius of the circle. 
 An equilateral triangle has its center on the perpendicular  bisector. The formula is a/sqrt(3) 

 r = 18/sqrt(3)
 = 18sqrt(3) / 3
 = 6 sqrt(3) 

The inscribed angle by the side is 60 degrees

so the central angle = 120 degrees

length of arc = {{{(theta/360) * 2*pi*r}}}

={{{(120/360)* 2*3.14*6sqrt(3)}}}

=21.75 cm