ANSWER
Subtract 3-1
EXPLANATION
The given line segment has endpoints at (1,6) to (3,6).
This line is a horizontal straight line because the y-coordinates of both points are the same.
We can therefore use the number line method to find the length of this line segment.
Using this method, we subtract the initial x-value from the terminal x-value.
The best description is:
Subtract 3-1