ANSWER
![x= 9 \: in](https://img.qammunity.org/2020/formulas/mathematics/middle-school/igk8ihpb4hda8mz55vf6t745ojxd8zb794.png)
EXPLANATION
The volume of a rectangular prism is given by the formula,
![V = l * b * h](https://img.qammunity.org/2020/formulas/mathematics/middle-school/fkyai7juuubut0vj9ffsbd44vofgt0ztcc.png)
We substitute the dimensions to obtain,
![V = 6 * 3 * 12 \: {in}^(3)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/txoidtn9pftbpw079ii1ail413kixb7sp5.png)
The volume of the rectangular pyramid is,
![V = (1)/(3) * 6 * 12 * x \: {in}^(3)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/bkaszlfhvmpd2qn79umnrnlromm0jyaaz7.png)
Equating the two volumes gives,
![(1)/(3) * 6 * 12 * x = 6 * 3 * 12](https://img.qammunity.org/2020/formulas/mathematics/middle-school/vkfd70wmdbud7bm0et8qepda28tqglsrog.png)
We divide through by 6×12 to get,
![(x)/(3)=3](https://img.qammunity.org/2020/formulas/mathematics/middle-school/qcactsm4q6h7a7tr73b9sbgr9v7wccz7zo.png)
We solve for x to obtain,
![x = 3 * 3](https://img.qammunity.org/2020/formulas/mathematics/middle-school/pdkroygxfqc8vp2nxx6jnfpwmruh4xgr3c.png)
![x = 9 \: in](https://img.qammunity.org/2020/formulas/mathematics/middle-school/47a7mgylgelbjudknypnx81ewb6cqelafq.png)