Determine whether the given vectors are orthogonal, parallel, or neither.
(a) $ a = \langle 9, 3 \rangle $ , $ b = \langle -2, 6 \rangle $
(b) $ a = \langle 4, 5, -2 \rangle $ , $ b = \langle 3, -1, 5 \rangle $
(c) $ a = -8i + 12j + 4k $ , $ b = 6i - 9j - 3k $
(d) $ a = 3i - j + 3k $ , $ b = 5i + 9j - 2k $