If my volume is 42.39 and my diamater is 3 what is the nswer

He basic equation for volume is length x width x height. When you have a cylindrical container I believe you have to take the area of the base x the height. 

Example - area(A) of the base equals Pi x (radius x radius) or half the diameter times itself then multiply that by Pi(3.14) 
Equation - A = Pi(r*r) 

Take the area and multiply it by the height of the fluid in the container. 

Equation - A*height = volume of a cylinder. 

If the cylinder is 3 inches across(diameter) and the fluid reaches 6 inches in height then my volume would be42.39 cu. in. 

A = 3.14(1.5*1.5) = 7.065 sq. in. 
7.065*6 = 42.39 cu. in. 

I hope this helps you. :)