Answer:
If Marco completely filled a box with 36 unit cubes with no gaps or overlap, then the box must have a volume of 36 cubic units.
The box could have different dimensions depending on how the cubes are arranged. For example, the box could be a cube with side length 3 units, since 3 x 3 x 3 = 27. Then, Marco could add another layer of 3 x 3 cubes on top of the first layer, giving a total of 3 x 3 x 4 = 36 cubes.
Alternatively, the box could have dimensions 6 x 1 x 6, since 6 x 1 x 6 = 36. This box would have two layers of 6 x 6 cubes stacked on top of each other.
There are many other possible dimensions for the box, but all of them must have a volume of 36 cubic units to accommodate the 36 unit cubes with no gaps or overlap.