The total area of the supply closet is the area of the original rectangle (length x question mark) minus the area of the removed rectangle (8 feet x unknown width).
Total area = (3 feet + question mark) x 2 feet = 36 square feet
Therefore, (3 feet + question mark) x 2 feet = 36 square feet
6 feet + 2x = 36 square feet
2x = 30 square feet
x = 15 feet
Therefore, the length of the side labeled with the question mark is 15 feet.