Given: mAngleEDF = 120°; mAngleADB = (3x)°; mAngleBDC = (2x)° Prove: x = 24

3 lines are shown. A line with points E, D, C intersects a line with points A, D, F at point D. A line extends from point D to point B in between angle A D C. Angle E D F is 120 degrees, angle C D B is (2 x) degrees, and angle B D A is (3 x) degrees.
What is the missing reason in step 3?

A 2-column table has 9 rows. Column 1 is labeled statements with entries measure of angle E D F = 120 degrees measure of angle A D B = (3 x) degrees measure of angle B D C = (2 x) degrees, angle E D F and angle A D C are vertical angles, angle E D F is-congruent-to angle A D C, measure of angle A D C = measure of angle A D B + measure of angle B D C, measure of angle E D F = measure of angle A D C, measure of angle E D F = measure of angle A D B + measure of angle B D C, 120 = 3 x + 2 x, 120 = 5 x, x = 24. Column 2 is labeled Reasons with entries given, def. of vert. angles, question mark, angle add. post., definition of congruency, substitution, substitution, addition, div, prop. of equality.