To find the length of the side of the supply closet labeled with the question mark, we need to first understand the information provided:
- The total area of the supply closet is 36 square feet.
- The remaining width on the left side of the rectangle is 3 feet.
- The remaining length on the right side of the rectangle is 2 feet.
From the description, it seems like we can consider the area of the supply closet comprised of the rectangle with the removed area.
The area of a rectangle is calculated as:
\[ \text{Area} = \text{Length} \times \text{Width} \]
In this case, we can set up an equation for the total area of the supply closet:
Let \( x \) be the unknown length labeled with the question mark.
So, the area of the closet can be expressed as:
\[ \text{Area} = \text{Width} \times \text{Length} = 3 \text{ feet} \times x \]
We know this area equals 36 square feet:
\[ 3 \times x = 36 \]
To find \( x \), we can divide both sides by 3:
\[ x = \frac{36}{3} = 12 \]
Thus, the length in feet of the side of the supply closet labeled with the question mark is:
\[ \boxed{12} \]