Hey there!
A right pyramid with a square base just means that it isn't slanted all funny. If you create a triangle with a point on one of the edges of the base, the center of the base, and the top of the pyramid, it would be a right triangle.
To find the volume of a right pyramid, you just take the base area, multiply it by the height, and then divide by three.
However, we are looking for the height. We have been given, so we will just go backwards.
252×3=756 (multiply instead of divide)
6×6=36 (base is a square, so you just square 6. This is our base area)
756/36=21
So, the vertical height of the pyramid is 21 cubic centimeters.
Have a wonderful day! :D