HTC Vivid: Screenshot Function not working. Help please

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http://forums.androidcentral.com – The screenshot function on my vivid is not working. Have watched several YouTube videos showing "how to" but still not being able to get it to work. Watched a video of a screenshot app for android and the trial Version did not work and the paid version gave a pop up that it wasn't licensed. I need a screen shot app or function. Help? (HowTos)