Try and try until you Succeed

You’ve probably heard this saying a billion of times , its what i’ve been trying to do since Thursday. I’ve been trying to solve this problem  and until now, i haven’t got the right answer yet. And its so frustrating  because i know that i’ve solved this problem before ( i lost it btw or maybe i threw it away ) and now, i’m having a very difficult time with it. We are having our second quiz tomorrow and since i messed up so badly on our first quiz, i really want to ace this one