The angle that measures 105 degrees is supplementary with the alternate interior angle of the angle which measures (3x - 18) degrees. Supplementary angles add to equal 180 degrees, so we can make an equation by first finding the measure of the alternate interior angle.
180 - 105 = 75
The alternate interior angle measures 75 degrees, so we can set that equal to 3x - 18 to solve for x since alternate interior angles are congruent.
