Section Modules will be used to compute the radii of a small plastic telescopic ball clamp I am designing. I am really stuck on this problem and appreciate you looking into it.

Section Modules is the resistance to bending of a beam. Section Modules, S for Circular tube is given by:

were r1 is inside radius and r2 is outside radius.

Section modulus - Wikipedia, the free encyclopedia

Here is the part I am stuck on:

Given S and r1, solve for r2.