The question needs some other information. The information is in the previous answer in a series of questions leading up to this final question. The immediately previous question is: "How much space does the helium take up?" The answer results in the volume information needed for the volume equation V=4/3(pi*abc). Because height and width (b and c) are equal, the equation reduces to V=4/3(pi*ac^2). Plug in the number for V (volume from the previous question) and for a (length) and then solve for c^2 (c squared).