Correct. The volume of two large cones is $\displaystyle{2}\cdot{144}\pi{{x}}^{{3}}={288}\pi{{x}}^{{3}}\usepackage{cmbright}\Huge\$ . The volume of two small cones is $\displaystyle{2}\cdot{18}\pi{{x}}^{{3}}={36}\pi{{x}}^{{3}}\usepackage{cmbright}\Huge\$ . The amount of empty space within the two large cones is the difference of these two volumes, which is $\displaystyle{288}\pi{{x}}^{{3}}-{36}\pi{{x}}^{{3}}={252}\pi{{x}}^{{3}}\usepackage{cmbright}\Huge\$ .