Suppose $PABCD$ is a right square pyramid with apex $P$ and base $ABCD$.  If $PBD$ is an equilateral triangle with side length 6, then what is the volume of $PABCD$?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Suppose $PABCD$ is a right square pyramid with apex $P$ and base $ABCD$.  If $PBD$ is an equilateral triangle with side length 6, then what is the volume of $PABCD$?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
18\sqrt{3}
Suppose $PABCD$ is a right square pyramid with apex $P$ and base $ABCD$.  If $PBD$ is an equilateral triangle with side length 6, then what is the volume of $PABCD$?
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