click the Extend button,
select Extend under the Modify Menu,
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.
Return to the AutoCAD Command List.