Question 144123
Since you asked a very general question, all I can do is give you a very general and probably very unhelpful answer.  <b><i> You do equations very carefully making sure that whatever you do to one side, you do to the other.</b></i>  Now, if you have some specific equations with which you are experiencing difficulty, write back with the specifics and tell us what you have tried.  Then we can give you some help.