A building in the shape of a rectangular pyramid is used to store construction stones. The building has a volume of 21000 ft^3. A new building is being designed. It will also be in the shape of a rectangular pyramid, but compared to the existing building, the length of the base will be scaled by a factor of 2/3 and the width will be scaled by a factor of 5/3. The height of the new building will be scaled such that the volume of the new building will be the same as the existing building, What scale factor will be used for the height of the new building? (Your answer should be simplified in fraction format.)