How to detect and plot lines on image?

</matlabcentral/answers/uploaded_files/116266/rrr.PNG> Hai. I want to ask, how can I plot a straight lines which detected by the edges? Then I can choose which lines that I want and control the line length. For example, I want the line that intersect at orange dot. So that I can calculate and display the angle. Can someone help me? I've done canny edge detection. I just need line intersections and angle calculation