**Quote:**

We could use the number substitution method to solve this problem.

Note, that for the substitution method to work in a "Yes/No" data sufficiency question we need to achieve the following:

1)

** In order to prove that information is not sufficient**, we need to plug in a value that yields "YES" as an answer, and a value that yields "NO" as an answer.

2)

**In order to prove that information is sufficient** you must plug in ALL the possible values and make sure that they all yield the same answer (always "YES" or always "NO"). Thus this method is seldom used to prove sufficiency.

**Quote:**

Since the above statement is incorrect, 1 is NOT SUFFICIENT.

You have shown that one possible value yields "NO" as the answer. But you know nothing about other possible values. If all the possible values yield "NO" as the answer, then this information will be sufficient.

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Since the above statement is incorrect, 2 is NOT SUFFICIENT.

Statement (2) is incorrect, because you've shown that one value yields "YES" and the other one yields "NO".

**Quote:**

To cross check this answer substitute the value of k as 4 in the the equation.

Substituting just two out of infinitely many possible values can not be a prove of sufficiency.

**Quote:**

Since the above condition hold to be true for all values of k > 2, both 1&2 together ARE SUFFICIENT to solve the question.

You have not shown that the answer is "YES" for ALL the possible values of k (positive even integers).

Try to feel the logic of a "YES/NO" data sufficiency question and how the number substitution method should be used here (to prove insufficiency mostly).