The line is extended from the end of the existing line that is closest to the selection point.
Click Home tabDraw panelLine drop-downCreate Line By ExtensionFind.
Select the line to extend.
To define the length of the line, do one of the following:
Select two locations to specify the length that you want to add to the line.
Enter a positive distance to extend the line.
Enter a negative distance to shorten the line.
Enter Total, or T, and then enter the total length of the segment. You can either type the new total length, or select two locations to define the total length. This total length can be greater than (to lengthen the line), or smaller than (to shorten the line), the current length of the line.