Step One
Find the base area of the large hexagon as though the smaller one was not removed.
Area = 3*Sqrt(3) * a^2 /2 where a is the length of one side of the hexagon
a = 5
Area = 3*sqrt(3) * 25/2 = 75 sqrt(3) / 2 of the large hexagon without the smaller one removed.
Step Two
Find the area of the smaller hexagon. In this case a = 4
Area2 = 3*sqrt(3)*16/2 = 3*sqrt(3)*8 = 24 sqrt(3)
Step Three
Find the area of the thick hexagonal area left by the removal of the small hexagon.
Area of the remaining piece = area of large hexagon - area of the small hexagon
Area of the remaining piece = 75 *sqrt(3)/2 - 24*sqrt(3)
Step Four
Find the volume of the results of the area from step 3
Volume = Area * h
h = 18
Volume = (75 * sqrt(3)/2 - 24*sqrt(3))* 18
I'm going to leave you with the job of changing all of this to a decimal answer. I get about 420 cm^3
