Click the Trim/Extend button if any entities are already in the Selection Set they are deselected and the prompt reads:
Give Arc/Line to Trim/Extend:
Select the line or arc near to the end to be modified.
Give Point to Trim/Extend to:
At this point the Snaps and Input Options change to display either or depending upon the last use. As the cursor is moved the line or arc length is adjusted to a point perpendicular from the entity to the cursor position. If Distance is set you can drag the cursor to the new position and click when the dynamic dimension displays the required adjustment, alternatively type the length directly into the input box and click anywhere in the drawing area to accept.