(Actually, now that I look at it, I'm not sure this answers your question at all. You've just been given a liner equation, and an uncertainty... Usually linear equations are accompanied by a correlation coefficient, not an uncertainty. )
I was asking some questions along this line some months ago
http://www.physicsforums.com/showthread.php?t=525091
The main thing that ought to be distinguished is systematic errors and random errors. You might not have any "random error" in your x-values, because you are measuring the same x-value in the same way, and getting the same result.
However, there may be a systematic error in your measurement of your x-value.
A quick google search gave me this article which gives some more detail:
http://www.ece.rochester.edu/courses...ncertainty.pdf
I'm not entirely certain how to account for possible systematic errors... You can really only account for systematic errors you KNOW are present.