### Instruction: 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 $PABCD$?### Response: I notice that the problem gives me the side length of the equilateral triangle $PBD$, but not the side length of the square base $ABCD$.
I also notice that the problem says the pyramid is a right pyramid, which means that the segment $PA$ is perpendicular to the base $ABCD$.
This means that the height of the pyramid is the same as the altitude of the triangle $PBD$.
To find the altitude of the triangle $PBD$, I can use the fact that it bisects the base and forms two right triangles with hypotenuse 6 and base 3.
By the Pythagorean theorem, the altitude of the triangle $PBD$ is $\sqrt{6^2 - 3^2} = \sqrt{27} = 3\sqrt{3}$.
This is also the height of the pyramid.
Now, to find the side length of the square base $ABCD$, I can use the fact that the diagonal $BD$ is 6 and forms a right triangle with two congruent sides of the square.
By the Pythagorean theorem, the side length of the square is $\sqrt{6^2/2} = \sqrt{18} = 3\sqrt{2}$.
The area of the square base is then $(3\sqrt{2})^2 = 18$.
The volume of the pyramid is one-third times the area of the base times the height, by a well-known formula.
Therefore, the volume of $PABCD$ is $\frac{1}{3} \times 18 \times 3\sqrt{3} = 18\sqrt{3}$.