Acone has a diameter of 18 units and height of 8 units. what is its volume?

The radius of your cone is half of 18 units = 9 units.

The volume of any cone is      V = (1/3) · (pi) · (radius)² · (height) 

-- Take that formula for the volume of any cone.
-- Write '8' in place of 'height'.
-- Write '9' in place of 'radius'.
-- Write '3.142' in place of 'pi'.
-- Complete the arithmetic.
-- There's the Volume :)