To answer this question we will use that alternate interior angles ( for parallel lines) are congruent and that the interior angles of a triangle add up to 180 degrees.
Using the fact that the interior angles of a triangle add up to 180 degrees, we can set the following equation:

Now, since a||b and angles 9 and 1 are alternate interior angles then:

Therefore:

Substituting

in the first equation we get:

Solving for x we get:

Answer: Option C.