The three views of your 2x6x2 solid will be 2x6, 6x2, or 2x2, with respectively 12, 12, or 4 blocks showing.

The three views of your 3x5x2 solid will be 3x5, 5x2, or 2x3, with respectively 15, 10, or 6 blocks showing.

If your premise is correct, the "top" could be any of the faces of the 3x5x2 solid. And the corresponding "top" of the other solid will be one, two, or all three faces, depending. Possible difference numbers are

15 - 12 = 3 (for any of the 6x2 faces)

15 - 4 = 11

10 - 4 = 6

6 - 4 = 2

Your answer could be any of 2, 3, 6, or 11, depending on how you define the "tops" of your solids.

The three views of your 3x5x2 solid will be 3x5, 5x2, or 2x3, with respectively 15, 10, or 6 blocks showing.

If your premise is correct, the "top" could be any of the faces of the 3x5x2 solid. And the corresponding "top" of the other solid will be one, two, or all three faces, depending. Possible difference numbers are

15 - 12 = 3 (for any of the 6x2 faces)

15 - 4 = 11

10 - 4 = 6

6 - 4 = 2

Your answer could be any of 2, 3, 6, or 11, depending on how you define the "tops" of your solids.