If I understand the setup, you have two line *segments* of different lengths which a parallel and a known distance apart. Also you know the length of one of the segments. From that, you want to find the length of the other line segment.

If that's the problem, then it doesn't have a solution; it has an infinite number of solutions. The setup puts no restriction on the length of the other line segment (other than it being different from the other segment's length) - it could be anything (except for the other segment's length) .

Is there some additional information to this problem that you've left out?