If line c is parallel to line d, the measure of angle 9 should be the same as angle 11 according to the Corresponding angles postulate. If a transversal (line b in this case) intersects two lines (lines c and d) and the corresponding angles are congruent, then the lines are parallel.
So now since the m∠11 is 120 and m∠11 is congruent to m∠9 then we can solve for x.
m∠9 = 6x
m∠11 = 120
6x = 120
Divide both sides by 6 to get x.
6x/6 = 120/6
x = 20
The answer is then A.
