You basically plug in x = 20, which is the same as saying "replace x with 20". Then you perform arithmetic to simplify each expression. Both simplify to 90, so angle 1 and angle 2 are the same measure. These angles are alternate interior angles, so we'll use the converse alternate interior angle theorem to conclude the lines are parallel.
Note: the alternate interior angle theorem says that if we have parallel lines, then the alternate interior angles are congruent. So the converse goes in reverse of this.
