This will open the properties dialog box, generally located at the bottom of the screen. This box will take on many forms, depending on what is selected on the stage. The image you have shown is the properties of a graphic. Text will have different properties, as will a MC, etc.
thanks for your help,i did it,i dragged MC b in the new MC,then in the
"properties",i changed the MC into Graphic,and selected "play once",
know what? it worked perfectly!