To solve this problem, you just have to subtract the volume of the glass box from the volume of the shipping container. Volume is calculated by multiplying width by length by height.
The glass box's volume: 7 * 9 * 12 = 756 in³
The shipping container's volume: 10 * 10 * 15 = 1500 in³
The difference between these values: 1500 - 756 = 744 in³
744 in³ of space will contain Styrofoam peanuts.