Given:
Volume of room = 1296 cubic feet
One side of the square floor of the room = 12 ft
Radistor is ⅓ of the height of the room.
Let's find the area covered by the mirror.
Here, the room has the shape of a square prism.
To find the height of the room, apply the formula:

Where:
V = 1296 cubic feet
a = 12 ft
Let's solve for h:

The height of the room is 9 feet.
Given that the radiator is ⅓ of the height of the room.
We have:

This means the height of the radiator is 3 ft
Let's find the area covered by the mirror.
Apply the area of a rectangle.
Since the radiator covers the entire length of the room, the length the radiator covers is 12 ft
The height of the radiator is 3ft

Therefore, the area covered by the G mirror is 36 square feet
ANSWER:
36 square feet