6. [10 points] true or false? circle your answer and provide a justification for your choice. (a) t f: if system ax = b has two different solutions x1 = (1, 2, −1) and x2 = (1, 0, 1) then (1, 4, −3) is also a solution. (b) t f: vectors (1, 2, 3), (4, 5, 6) and (7, 8, 9) form a basis in r 3 . (c) t f: the columns of a matrix form a basis for its column space. (d) t f: a system ax = b has at most one particular solution.