Question 871926
Okay, understood.


The roof section is probably meant as the half spheric part.
{{{h*pi*r^2+(1/2)(4/3)pi*r^3=v}}} and your v is 18000 cubic feet.


...
{{{h*pi*r^2+(2/3)pi*r^3-18000=0}}}
{{{50*pi*r^2+(2/3)pi*r^3-18000=0}}}
{{{(2/3)pi*r^3+50*pi*r^2-18000=0}}}
{{{highlight_green(2*pi*r^3+150*pi*r^2-54000=0)}}}
{{{highlight_green(pi*r^3+75*pi*r^2-27000=0)}}}.
You are looking for the roots or zeros, so there should be one real value that should make sense.  This is a cubic equation, and maybe a graphing tool would be useful.