When you have parallel lines that are vut by a transversal line, you get a 2 pairs of 4 angles, this pairs are congruent or suplementary, this means that the sum of their angles is 180º or they are equal. So ig you have that one angle is 150º the other one since is suplementary must be 30º, since 150*30=180, and all of the angles are equal to these two, 150º and 30º angles.
