Setting The X Position Of A Movie Clip

How to set the horizontal stage position of a movie clip with actionscript:

• Create a movie clip and place it on stage (in the first frame of the main timeline).
• Click on the first keyframe in the main timeline then open the "Actions" panel.
• Enter this code in the "Actions-Frame" panel.
  

onClipEvent(load){
this._x = 400;
}

• Press Ctrl + Enter to test the movie.