recover icloud keychain forgotted device password
I change my login password and now I need to login again in iCloud
I tried to login from Settings -> iCloud -> add my iCloud password, my pc password and at the third step is asking for another pc's password to be able to access iCloud keychain ( screen attached ) but i don't remember that password nor I have that pc because I sold it
how can I pass this step?
thanks
Mac Pro, OS X 10.11