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 $PABCD$?The answer is in the form k\sqrt{m}+n,. Please provide the value of k + m + n.

Let's think step by step and output the final answer within \boxed{}.
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