we know that
The volume of a cone is equal to
[tex]V=\frac{1}{3}\cdot\pi\cdot r^2\cdot h[/tex]In this problem
we have
V=75pi mm^3
h=9 m------> convert to mm
h=9,000 mm
substitute in the given equation
[tex]\begin{gathered} 75\cdot\pi=\frac{1}{3}\cdot\pi\cdot r^2\cdot9,000 \\ \text{simplify} \\ 75=r^2\cdot3,000 \\ r^2=\frac{75}{3,000} \\ \\ r^2=\frac{1}{40} \\ \\ r=\frac{1}{\sqrt[\square]{40}} \\ \\ r=\frac{\sqrt[\square]{40}}{40} \\ \text{simplify} \\ r=\frac{2\sqrt[\square]{10}}{40} \\ \end{gathered}[/tex][tex]r=\frac{\sqrt[\square]{10}}{20}\text{ mm}[/tex]