Complex sentence: The population density was 1,439.2 people per square mile (555.7/km²) and 559.6 housing units per square mile (216.1/km²).
Split into simpler sentences: The population density was 1,439.2 people per square mile (555.7/km²). There were 18,769 housing units at an average density of 559.6 per square mile (216.1/km²).