Solve the equation by making an appropriate substitution. 40 2 X - X - 3 x - - 18 = 0 Make an appropriate substitution and rewrite the equation in quadratic form. 40 Let u = x- then the quadratic equation in u is u - 3u - 18 =0 X Solve the equation. Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. The solution set is (Simplify your answer. Use a comma to separate answers as needed.) O B. The solution set is the empty set