At first, push Volume Down and Power key for a few seconds.
If the screen goes dark hold down Volume Up + Power button for a couple of seconds.
When the Recovery Mode shows up release all keys.
After that select Wipe data/factory reset by using Volume keys to navigate and the Power button.
Then choose Factory data reset from the menu and confirm with the Power key.
Finally, select Reboot systemnow with the Power button.
Good job! You are performed hard reset operation.
After hard reset your SAMSUNG Galaxy A51 can be locked by Google Protection. If you would like to bypass this feature, check out our instructions: Unlock FRP SAMSUNG.