There are several methods you can use to extend the length of a line. Which one to use depends on how many lines you want extended at a time and whether you want the line extended to a sketch you draw or based on an intersection with an existing line. The main ways you can extend a line are by using the Extend/Trim edit task or the Extend tool on the Advanced Editing toolbar.

You can also extend line features that violate a "Must not have dangles" topology rule with the Extend option of the Fix Topology Error tool.

The Extend/Trim edit task lets you select a line and extend it as far as a line that you sketch. In this example you see how to set the edit task to Extend/Trim Features, select a line, and extend the line to sketch drawn along a building edge.