Walter puts 1050 cubic inches of dirt into the tank shell and draw a line to show the height of the dirt in the tank

That line could have an infinite possibilities of height, for example if the tank is cubic, 20 inches per side, then we can calculate the line's height:

volume = 20*20*height

given the fact that the tank has 20 inches per side, and we know the volume as a data given:

1050 = 20*20*heigth

hence the height is:

height = 1050/400

height = 2.63 inches