For which value(s) of kkk is the matrix A=(k14k)A = \begin{pmatrix} k & 1 \\ 4 & k \end{pmatrix}A=(k41k) non-invertible?
k=2k = 2k=2
k=−2k = -2k=−2
k=4k = 4k=4
k=−4k = -4k=−4