The Trim Position function can be used to trim or extend an entity to a position that you indicate.
Using the Function:
Select Position from the Modify>Trim submenu.
Select the entity that you want to trim or extend, on the side that you want to keep.
Indicate the position to which you intend to trim or extend the entity. The nearest point on the entity in the XY plane of the current CPlane is computed, and the entity is either trimmed or extended to that position. Because the entity and position are projected into the CPlane, entities that project into a single point in the XY plane of the CPlane, such as a line parallel to the Z axis of the CPlane, cannot be trimmed or extended using this function.