click the Extend button,
OR
select Extend under the Modify Menu,
OR
type extend at a Command: prompt and press Enter.
--Prompt: Current
settings: Projection=UCS Edge=None
Select boundary
edge(s)...
-Select objects: Click on the item(s) that you wish to use as a boundary.
The boundary is the object you want to extend an element TO (you can select
more than one boundary edge so that multiple extensions can be made at one time).
Press Enter or click the right mouse button to end the boundary
selection process.
--Prompt: Select object to extend or [Project/Edge/Undo]: Click on
the end of the line you want AutoCAD to extend (on the side that
should be extented). AutoCAD is directional in this command. If you have
more than one boundary edge selected, or the boundary edge could intersect the
object at more than one end (as with an arc), it is possible to extend the element
in the wrong direction.
--Prompt: Select object to extend or [Project/Edge/Undo]: If you
have other lines to extend, you can continue selecting them or, if an element
was extended incorrectly, you can type u and press
Enter to undo the mistake. After all of the extensions are complete, press
Enter to exit the command.