Can also try using real numbers, if that's easier for you to follow. This is a yes/no question, so you basically try to get both answers - if you can, it's insufficient. If you keep getting the same answer, though, then you can conclude it is insufficient.
(1) If k=2, n=7 and yes n is div by 7. If k=3, n=9 and no n is not div by 7. Insufficient.
(2) This does not give us a connection between k and n, so can conclude nothing about n. Insufficient.
(1) + (2) If k=2, 2k-4=0, which is div by 7. If k=2, n=7, so yes n is div by 7. If k=3, 2k-4=2, which is not div by 7, so k cannot equal 3. Try some others and see if you can figure out a pattern. If k=4, 2k-4=4, so can't use k=4. If k=5, 2k-4=6, so can't use k=5. Every time I increase k by one, I increase 2k-4 by 2. So the next one that will work is k=9. 2k-4=14, so I can use k=9. If k=9, n=21, so yes n is div by 7.
Note that using this method, you aren't 100% sure - it's more that you've tried enough numbers to find a pattern, so you extrapolate. But this can be easier than the official math way - so just have to make a choice.
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