If you have set up Lock Pattern to password protects your Nexus 7 tablet and forget which dots to use to unlock, that will be a big problem. You need to try to use the combination possible five times. If you still can't remember the screen unlock pattern, only one solution is do hard reset your tablet.
* In the Drwa pattern to unlock" screen, touch on the "Forgot password" button.
* Enter your Google or Gmail account, which is used when you first register or activated Nexus 7, to sign in. Then do a reset and your personal data and contacts stored on your tablet memory will be lost.