Is there some convention or common sense (which I appear to lack) when it comes to choosing which sides you will assign to which variables? I assumed it didn't matter, but I've come across many cases where I come up with very different answers.

For instance, in problems where 3 sides are given I was provided 5, 11, and 9 as the length of the sides. If I say a = 11 b = 9 and c = 11 I wind up with the incorrect answer but if I use 9 for a, 5 for b and 11 c then I can get the correct answer.

Can anyone explain to me how I'm supposed to know which sides to use for a, b and c?