6/9 is equivalent to the fractions 60/90 and 600/900. The amount of zeros you add on does not matter as long as you do it to the top and bottom. The division sign cancels it out.
It has to do with the number of dimensions you have in the figure. For example, in a square you have 2 dimensions, length and width. Since you have only two you use units^2. In a cube, you have 3 dimensions, length, width and height, which is where you get the units^3.
You have to find what x is to do all of them
