You can extend a straight line at the end of an open object with the Parallel and Perpendicular line buttons. The added line maintains the slope of the original line, or is at a right angle to the original for the Parallel and Perpendicular buttons, respectively.
To extend a straight line, double-click the line you want to extend, click the Simple Line Tool in the toolbox, click the Line Segment button in the ribbon, and click the Parallel or Perpendicular button. Point to the end of the line you want to extend and drag the extended line.