To convert the real-life length of 15 feet into the length in the diagram, we first need to determine how many 5-feet segments are in 15 feet.
15 feet divided by 5 feet gives us:
\[ \frac{15 \text{ feet}}{5 \text{ feet}} = 3 \]
This means that 15 feet corresponds to 3 segments of 5 feet. Since each segment of 5 feet is represented as 2 inches in the diagram, we calculate the total length in inches by multiplying the number of segments by the corresponding length in the diagram:
\[ 3 \times 2 \text{ inches} = 6 \text{ inches} \]
Therefore, Trent's living room should be 6 inches long in his diagram.