やられたらやり返す、倍返しだ (yarare-tara yari-kaesu, bai-gaeshi da)
If I’m attacked, I’ll counter-attack. I’ll return it in double (It's double-returning).
やる: to do (casual), to attack
たら: if (conjunction, attached to the verb)
返す: to return
→やり返す: to counter-attack
倍: double (noun)
返し: returning (the noun form of 返す)
だ: be