C= 2pir solve for r
me with this

C = 2* pi * r ;

is the formula for the circumference of the circle;
                      in which:
                                      C = the circumference of the circle;
                                      pi , or " π " ;  is the symbol for "pi" ; 
                                      r = the radius of the circle ; 

To solve for "r" (the radius), divide each side by "2*pi" ;
         to isolate "r" (the radius) on one side of the equation;
        and to solve for "r" (the radius):

C = 2*pi*r ;

C / (2*pi) = {2*pi*r} / (2*pi) ;

to get:

C / (2*pi) = r ; 

↔ r = C / (2*pi)  .

Yes i could be the length of the triangle

21/2 or 5/2