1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution Obviously I don't need a solution because it's right there. What I need to understand is what happened after the third equation sign and more importantly, how would I learn to solve these kinds of problems on my own. I looked at Wikipedia and even though it was helpful, I still don't understand this specific exercise for example. I would be grateful if someone could point me at a resource on this topic.