Volume is determined by multiplying the height, length, and width together.
Therefore, you can use the formula:
V = lwh
Values given by you:
L = 56
W = 12
H = 13
So, you need to substitute these values into the formula, then solve.
V = lwh
 = (56)(12)(13)
 = (672)(13)
 = 8736
Therefore, 8736 yd^3 of sand is required to fill the box.
 ^
 Because volume is measured in cubics, ^3.