1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I just started integration. This is the first example from the lesson. I get to the part where they take the derivative of u with respect to x. But the next step, where it says dx=du/6x, I'm totally lost. I don't understand how they got that, or what it means, or even how it is read.