Lines extend past shape when using shape builder tool to color


I also cannot seem to understand joining lines, im takin a little class and the teacher made this triangle. I did not see him have to manually join any lines, he just had nice corners after using the shape builder tool. I had to manually join them because it left gaps like this:

maybe that is the reason they are extending past the shape. Im just gettin real stuck here would really appreciate any help! Here is the dropbox link to download the file.
