The model below illustrates what problem? Image of a number line . . . spanning from negative 4 to positive 4 in increments of 1. A line starts at 1 and ends at 3. A second line starts at 3 and ends at negative 3.Image of a number line . . . spanning from negative 4 to positive 4 in increments of 1. A line starts at 1 and ends at 3. A second line starts at 3 and ends at negative 3.