Determine whether the given vectors are orthogonal, parallel, or neither.

(a) a = (9, 3),    b = (−2, 6)

(b) a = (4, 5, −2),    b = (3, −1, 5)

(c) a = −8i + 12j + 4k,    b = 6i − 9j − 3k

(d) a = 3i − j + 3k,    b = 5i + 9j − 2k