Make 'r' the subject of the formula: v = 4r²h

v = 4π r²h is to be solved for r.

Divide both sides of this equation by 4π*h:

     v

= r^2

   4π*h

Next, take the positive square root of both sides:

              v

r = √[ ]

           4π*h

You may leave this result as is, or proceed further:  Taking the + square root of 4 results in:

                      v

r = (1/2)√[ ]

                    π*h

because they could drop even lower..