Lines do not have a finite length, hence it is impossible to have two lines of unequal length, parallel or otherwise.
Now, if what you really meant was "...two parallel line segments of unequal length," that is another story.
Now the problem is that it is not clear whether you are talking about some existing line segment between the two parallel lines or some hypothetical line segement that you are supposed to construct between the two parallel lines, probably a perpendicular.
I also don't know what other information you have. In either case above you have to either be given or be able to derive the coordinates of the points where the line segment whose length you need to calculate intersects the two parallel lines. Then use the distance formula:
where and are the coordinates of the intersecting points.
John
My calculator said it, I believe it, that settles it
