How do you find a missing angle simply

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

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