Action Script commands

Hi experts

I have this code:

this.createEmptyMovieClip("line_mc", 1);
line_mc.lineStyle(2, 0x000000, 100);
line_mc.moveTo(350, 200);
line_mc.lineTo(500, 200);
line_mc.lineTo(500, 400);
line_mc.lineTo(350, 400);

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How i can switch between visible and invisible?
How I can destroy it when I dont need it?

  Thank you in advance.

  Regards,
  Raul M.