Answer:
your answer depends on what sides are given.
For example, lets say that you now the lengths of two sides, these being 56 and 24. ( these are just random numbers BTW) to find the third you would add the two lengths that you know and then minus them from 180. So for the example i just typed you would do 56 + 24 first which equals 80, then would do 180-80 which equals 100.
To check your answer, you could do this:
DO THE ABOVE AGAIN CAREFULLY (lol I don't usually check so.. idk)
math is my worst subject so.... hope this helped you.