To find the missing length \( x \) in the diagram described, we first note that the total area of the rectangle is given as 300 square feet, and the width of each section is consistently stated as 15.
The area of a rectangle is calculated with the formula:
\[ \text{Area} = \text{Length} \times \text{Width} \]
In this case, the total area can be represented as the sum of the areas of the three sections together. The lengths of the first and third sections are provided, while the second section has an unknown length \( x \).
The equation representing the total area in terms of the sections is as follows:
\[ \text{Total Area} = \text{Width} \times (\text{Length of Section 1} + \text{Length of Section 2} + \text{Length of Section 3}) \]
Given that the width is \( 15 \), the equation becomes:
\[ 300 = 15 \times (6 + x + 4) \]
Simplifying this equation gives us:
\[ 300 = 15(10 + x) \]
Thus, the correct equation to calculate the missing length \( x \) based on the diagram described is:
\[ 15(6 + x) = 300 \]
Therefore, the correct response is:
15(6+x)=300