HOW DO YOU ERASE

Which program were you using when you drew the line? Most programs offer an "undo" option under the "Edit" menu -- this may work here.

Assuming you are using Inkscape and that you want to modify a line object you drew to be shorter instead of completely remove it, then you should be able to do what you want by adjusting the nodes of the shape. There is a lot of helpful information on this and other topics in the Inkscape tutorial at:

http://ostan-collections.net/annex/InkscapeTutorial.pdf

In 0.47 there is a path eraser tool which can be set to either delete whole path on touching them (not useful IMO and shouldn't be the default) or you can set it to cut out a particular width of line from a path, which is like using a regular eraser. The ultimate effect is as if you drew a thick line and converted it to a path then subtracted it from your image (Path > Difference), quite handy.