Well this is how you would do it:
It is a square sandbox, meaning it is 7*7*7, but she only wand the sand to be 2 feet high, so you would remove 5 ft from the box's height. The new volume would be 7*7*2
So the amount of sand would be equal to the new volume!
Therefore she would need to order 98 ft^3 of sand
I hope I helped!