It's the formula of the volume of a cone with equal lengths of radius and height.
[tex]\dfrac{1}{3}\pi r^3=V\ \ \ \ |\cdot3\\\\\pi r^3=3V\ \ \ \ |:\pi\\\\r^3=\dfrac{3V}{\pi}\to r=\sqrt[3]{\dfrac{3V}{\pi}}[/tex]
