what line of longitude forms the other half of the prime meridan?

Answer 1

The longitude that lies opposite to the Prime Meridian is the 180 degrees EW longitude, also called the International Date Line.

Answer 2

Longitude 0° forms the entire prime meridian. A meridian is half a circle and runs from the North Pole to the South Pole. I think you mean, "What other line of longitude completes the great circle that contains the prime meridian?"

The answer is 180° longitude. 180°E longitude and 180°W longitude are the same meridian. The international date line more or less follows this meridian with a few zigs and zags away from it here and there.

Answer 3

180 degrees, either east or west (they are the same), is one half way around the world from the prime meridian, and completes the circle of that meridian around the globe