Question 59544
The goal is to get the r all by itself.  To get rid of the things with it you do the inverse operation to it. (If it's multiplied-divide it and if it's squared-sqrare root it.)  Whatever you do to one side of the equal sign, you must do to both sides.
{{{V=(pi)*r^2*h}}} solve for r.
{{{V/((pi)*h)=(pi)*r^2*h/((pi)*h)}}}  because a/a=1
{{{V/((pi)*h)=r^2}}}
+/-{{{sqrt(V/((pi)*h))=sqrt(r^2)}}} because {{{sqrt(a^2)=a}}}
{{{sqrt(V/(pi*h))=r}}} 
You can ignore the +/-, because Volume is positive.
{{{highlight(r=sqrt(V/(pi*h)))}}}
Happy Calculating!!!