In each case, find all [a b c d]T m R4 such that the given set is orthogonal.
(a) {[1 2 1 O]T, [1 -1 1 3]T, [2 -1 0 -1], [a b c d]T}
(b) {[1 0 - 1]T, [2 1 1]T,[l -3 1 0]T, [a b c d}T}