When the height decreases, it becomes a cube with the same length and width, and let it be X.

The original surface area is 2*X*X+4*X*(X+3).

Later, the surface area was 6 * x * x

The subtraction between them is 12X= 144.

There is X= 12.

The original surface area is 1008.

Or maybe.

The surface area of a cuboid is

Two bottom areas+four side areas.

After high reduction

The area of that two bottoms are unchanged,

What is missing is the surface of the high part.

Let the length and width be x.

There are 4*3*X= 144

So X= 12.