The straight line Y = 4/3x is shifted downward by 6 units to get Y = 4/3x-6. When Y = 0, X = 9/2, so the coordinate of point C is (9/2,0). Let AC be perpendicular to the X-axis and BD be perpendicular to the X-axis, then △AOC∽△BCD, and let A(a, b) and B(c, d) be in direct proportion to the corresponding edges, so that A: (b-9/2) = 2, C: D = 2, so A = 2b-9, C.