Where did you get this problem if you do not even know the notation? I cannot imagine a text book using the notation $\displaystyle ||x||_2$ without first having defined it!
Where did you get this problem if you do not even know the notation? I cannot imagine a text book using the notation $\displaystyle ||x||_2$ without first having defined it!
No, it isn't from a text book ... I was just trying to solve some problems from an online website "brilliant"