The radius of the cone will reduce linearly from the Base to the mid section proportionately with the associated height. Therefore at the mid section the radius of the upper cone will be 3 units.

So required volume will be just the difference between the volume of the 2 cones

Ie.,

((pi/3)*r1^2*h1) - ((pi/3)*r2^2*h2)

Here r1= 6, h1 = 24, r2 = 3, h2 = 12.

Therefore we get the answer as = 252*pi