If the sum of two angles is 180 degrees then they are said to be supplementary angles.
and if the sum of two angles is 90 degrees, then they are said to be complementary angles.
So
Supplement of 120 degrees = (180-120) degrees = 60 degrees
and
complement of 60 degrees = (90-60) degrees = 30 degrees
So the complement of the supplement of 120 degrees is 30 degrees