120 and the angle next to it are supplementary angles meaning they add up to 180. So take 180 - 120 to get 60. A triangle's angles add up to 180. So you take the 60's from the 180 to get the other sixty. In other words 180 - (60 + 60) = 60.
Hope this helped. I feel like I explained this kinda weird so ask questions if you have them.
