I got the same question on Khan Academy. I was going to ask that too but saw that you already did. Maybe try to point it out to someone else on Brainly. Sorry that I could not help.
Supplementary angles will always add up to 180. If you had a straight line, and you drew another line across that line, the two angles on either side of that line would add up to 180 because a flat line is equal to 180 degrees.