use the reference sub command in the scale command.  draw a line that is the length you neet to scale to.  Place the end of that line on the end of the orginal line you are referencing to. Make sure they are going the same direction.  type SCAE to start the scale command.  Select everything you are going to scale (you may want to unselect the longer line) and press enter.  Select the matching end point of both lines as your base point.  Now type R for reference.  Select the SAME point you just selected, then select the opposite end of the shorter line.  now select the end of the longer reference line.  Kind of a long process written out, but once you do it once it's a lot simpler than it reads.