What is parallel lines?

parallel lines means two distinct lines have the same slope.

in this problem, although they have different ranges, but they representing the same line (line means extending the line segment to infinity).

so they are not parallel

Parallel LInes: the two lines do not meet from the beginning to the infinity (無限不距離相遇).

Obviously not.
the lines are both y=3x, so they are straight lines, although x has different ranges.