Find all values of k for which the following homogeneous systems of linear equations have a nontrivial solution:
(a) x + k y = 0
kx + 4y = 0
(b) x1 + kx2 + 4x3 = 0
kx1 + X2 + 2x3 = 0
2x1 + kx2 +8x3 = 0
(c) x + ky+2z = 0
3x -ky-2z = 0
(k + l)x - 2y - 4z = 0
kx + 3y + 6z = 0