Thanks in advance for the help :)
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To be always negative, the parabola must be open down. So, k < 0.
Now apply discriminant, b^2-4ac < 0
4-4k^2 < 0
k^2 > 1
Answer: k < -1
Now apply discriminant, b^2-4ac < 0
4-4k^2 < 0
k^2 > 1
Answer: k < -1