Solve for r: C = 2 (pi) r.

the main condition can be found one of the following given methods: if A = B*C*D, if we want to find the value of B, so B = A / C*D, the similar method can be applied to find the value of C an D. These are C = A / B*D and D = A / C*B in our case we have C = 2*Pi * R, solve for R does mean, searching the value of R, so if we want to find the value of R, it is R = C / 2*Pi