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