For (iii), T(v_1) = v_1 and T(v_2) = v_2, since the projection of vectors lying on a plane onto the plane is that vector itself. Taking co-ordinate vectors of T(v_1) and T(v_2) w.r.t. the ordered basis will give (1, 0, 0)^T and (0, 1, 0)^T as the first two columns. Then, since v_3 is orthogonal...