My "brain cells that handle 3D" are weak, few, and far apart.

But here's what I'd do:

1: buy a perfect cube of soft cheese (on sale)

2: turn the 9*9*9 and 3*3*9 to 3*3*3 and 1*1*3

3: "line" the surfaces so that each looks like a "X and O" grid

4: going top to bottom and using appropriate kitchen knife,

remove the vertical 1*3 at center; so three 1*1's have been removed

5: perform similar surgery both ways horizontally; here one must remember

that only two 1*1's are removed for each (the center one is gone from step 4),

so four 1*1's have been removed

a = surface area of initial cube: 6 * 9 = 54

b = "removed area"; a 1*1 from each surface: 6 * 1 = 6

c = "added area"; that's 4 surfaces of six 1*1's: 6 * 4 = 24

So exposed area = a - b + c = 72

Now translate that to a 9*9*9.

Well, hope that helps a bit...