You can unlock your tablet in 2 ways. You can perform a factory reset of your tablet, or you can use your Google account to unlock it. To perform a factory reset, you need to turn it OFF first. After that, hold pressed the Volume Down and Power buttons until the Android logo appears on the screen. Then you can use Volume down and Volume up keys to navigate through the recovery menu. To reset your tablet, select the Wipe data / Factory reset option. Be advised, by doing this, you will lose all the data from the tablet.
You can also unlock your tablet by inputing a few times a wrong code until a message that ask you if you forgot your password appears. Tap on it and follow the instruction provided. By doing this, you will use your Google mail account to reset the password from your tablet. In this way, you will keep all the data that you currently have on your device.