The Trim First function can be used to either trim or extend the first entity that you select.
Using the Function:
Select First from the Modify>Trim submenu.
Select the entity to trim or extend on the side that you want to keep, near its intersection with the trimming entity.
Select the entity to trim or extend near its intersection, or projected intersection, with the first entity. The entity is trimmed or extended. The side you selected to keep in step 2 will remain.