The surface area, S, of a sphere is given by a formula S=4pi r squared. Make r the subject of the formula.
$${\mathtt{S}} = {\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{r}}}^{{\mathtt{2}}}$$
$${\frac{{\mathtt{S}}}{\left({\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}}\right)}} = {{\mathtt{r}}}^{{\mathtt{2}}}$$
$${\mathtt{r}} = {\sqrt{{\frac{{\mathtt{S}}}{\left({\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}}\right)}}}}$$
.$${\mathtt{S}} = {\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{r}}}^{{\mathtt{2}}}$$
$${\frac{{\mathtt{S}}}{\left({\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}}\right)}} = {{\mathtt{r}}}^{{\mathtt{2}}}$$
$${\mathtt{r}} = {\sqrt{{\frac{{\mathtt{S}}}{\left({\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}}\right)}}}}$$
