I'm sorry math, but I'm not seeing how the quadratic formula has anthing to do with solving for k. Now, if we were solving for x, then the quadratic formula would be most appropriate.
K is not quadratic at all. In fact it is linear (or even more probable; a constant).
But for x...
The quadratic formula states that
Let a=1, b=-(k+1), and c=(k+1)
You can do the rest...
Is this the whole question, exactly as it's written in your book (or wherever it came from)? I doubt it very much since I'll bet diamonds to doughnuts that the correct answer is k = -1, 3. Next time please post the whole question so that people don't waste their time.