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.
-- Bada-bing, you've got the volume.