Final answer:
True, longitude is measured using the Prime Meridian as the zero-reference point, and it determines if a location is east or west of this line.
Step-by-step explanation:
The statement that you must use the Prime Meridian as the zero-reference point when calculating longitude, determining whether a location is east or west of this meridian, is true. Longitude is the number of degrees of arc along the equator between your meridian and the Prime Meridian, which passes through Greenwich, England.
The Prime Meridian has been internationally agreed upon as the starting point for longitude measurements, designated as 0°. Locations east of the Prime Meridian are denoted by degrees east (E), and locations west are denoted by degrees west (W).