How do you know where to put the parentheses in an equation?

Parentheses are used to identify which numbers you should add,multiply .ect first. So If i had

12-(3+1)=x    

then you would want to solve 3+1 before you subtract 12.

So its like you have an answer to a problem and put the parentheses 

all you need to do is just try out different combinations of n \umbers to do first and then do the number(s) outside

hope I helped :D