Select the Line tool (1)

Then select the Line Parallel (2) option, and then enter in the distance away (3) you want the parallel line.  (for 2.5 inches, enter 2.5")

Then select a line in the drawing (4), and move the mouse to one of the sides of the line.  It will then place the next line the set distance away (5).

Trim or extend lines to another line

Select the Modification Tool (1)

Then select the Trim Tool (2)

Then we want to extend one line (a) down to another (b), and then we want to remove all the excess (c) part of (b).

To first extend the line, select the line we don't want to change (3), and then select the line you want modified (4).  Notice how the line we clicked second is extended to the line we clicked first?  

Now to remove the excess line, select the line we don't want to change (5), and then select the side of the line that we want to keep (6), and the rest of the line will be deleted.  

Here's a bunch more trimming examples.  First click the line you don't want to modify (1), and then the one you do (2).

- QCAD Book. PDF instructions manual for the application.