Final answer:
Trinity can use the equation V = (x-6)(x-6)(3) to determine the volume of the box.
Step-by-step explanation:
The equation that Trinity can use to determine the volume of the box is V = (x-6)(x-6)(3). This equation represents the volume of the box, where x is the side length of the original piece of cardboard. By subtracting 6 from each side length of the cardboard (to account for the 3 in. squares cut out from each corner) and multiplying by the height of the box, Trinity can calculate the volume of the box.