With the Trim/extend function from the right mouse button menu it is possible to trim or extend lines. To use this feature:

1. select the endpoint of the line that is to be extended or trimmed;
2. add to the selection the extension target line (or trimming boundary line), e.g. using CTRL+Click;
3. right click and select the Trim/extend option from the pop up menu:
   ;
4. the geometry will be updated accordingly.

Two examples are shown below, and in the animation below:

Note that using this option through the command line, the following command should be used: snaplinear.
It takes two arguments: first the endpoint of the line to be trimmed, and secondly the line it should trim/extend to, for example:

Also see the command reference from the Help menu for further information.

Animation: extending and trimming a line