How do you find a missing angle simply

If you have a rombus for example lets say the obtuse is 180 and the acute is 30 so the opposite angel of both is what the other is if that makes any sence

Lets say you have a triangle that is 180 one side is 90 another is 56 and the other is blank so what you need to do is add 90+56=146 and then do 180-146=34 so then 90+56+34=180, 34 is the missing angel, is that what you need