To find the length of the side of the supply closet labeled with a question mark, we can use the formula for the area of a rectangle, which is:
\[ \text{Area} = \text{Length} \times \text{Width} \]
We know the total area of the closet is 36 square feet, and the width is 2 feet. Let \( L \) be the length of the closet. We can set up the equation:
\[ 36 = L \times 2 \]
To find \( L \), divide both sides of the equation by 2:
\[ L = \frac{36}{2} = 18 \]
Therefore, the length of the side of the supply closet labeled with a question mark is
\[ \boxed{18} \text{ feet.} \]