Question 821251
C=(pi)(2r)
A=(pi)r^2

Solve for r in the second equation.
A/(pi) = r^2
Take square root of each side.
sqrt( A/(pi) ) = r
Substituting in C=(pi)(2r) we have
C=(pi)(2( sqrt( A/(pi) ) )) or
C={{{(pi)(2( sqrt( A/(pi) ) ))}}}
A in the equation for our problem is l x w = 12*6 = 72
so
C={{{(pi)(2( sqrt( 72/(pi) ) ))}}}
 = {{{(pi)(2*6( sqrt( 2/(pi) ) ))}}}
 = {{{(pi)(12( sqrt( 2/(pi) ) ))}}}