More information please. Android 6 is only just starting to roll out so you won't have Android 7. What tablet do you have and what version of Android is it running? You can find this in settings>about device.
Android 7? There is no Android 7 yet, unless you have a custom ROM named "Android 7" The latest and greatest is Android 6.0.1 Marshmallow. I'm on 5.0 and to take a screenshot on my phone is home button + volume up button.