18. let e = {u1, u2, u3} and f = {b1, b2}, where u1 = ⎧⎩ 1 0 −1 ⎫⎭ , u2 = ⎧⎩ 1 2 1 ⎫⎭ , u3 = ⎧⎩ −1 1 1 ⎫⎭ and b1 = (1,−1)t , b2 = (2,−1)t for each of the following linear transformations l from r3 into r2, find the matrix representing l with respect to the ordered bases e and f: (a) l (x) = (x3, x1)t (b) l (x) = (x1 + x2, x1 − x3)t (c) l (x) = (2x2,−x1)t