Extending a line segment, arc, 2D polylines or rays, the tangent with another object. You can use the polyline, arc, circle, ellipse, building line, line, ray, spline or view a paper space as a boundary object. When you use a 2D polyline as a restriction object, the object will be extended up to the center line of the line segment.
1. Click on the menu bar "Edit> extension.
2. Click the Edit toolbar "extension order" button 
.
3. Enter the Extend command line, and press Enter.
After the execution command, the command line displays the following information:
Command: Mx_Extend
Select Object <select all>:
Select an object to extend:
Select an object to extending: Specify opposite corner: