Should we solve this problem using +3 only? Can we not consider -3 as another root and proceed further?
You are asked to compare quantites k and -1. If you are told that 3 is a root of the equation, then the only thing we need to do is plugging in the 3 in order to get that k=-1. After this, we have at hand the answer to our question. Now, of course if you know that k=-1 you can then say that x^2-x-6=0 and so the second root of the equation is -2 (not -3 and you stated). However, is it at all helpful to know the second root? Note that we first needed the value of k in order to find the second root.
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