For which kkk is ⟨2,k,−1⟩\langle 2, k, -1 \rangle⟨2,k,−1⟩ a unit vector?
k=0k = 0k=0
k=±2k = \pm 2k=±2
No real value of kkk works
k=±−4k = \pm\sqrt{-4}k=±−4 (imaginary)