1. The problem statement, all variables and given/known data It's been god knows how long since I've had to use integration by substitution. I've totally forgotten it. I am trying to integrate to solve for the value of an electric field at a given point. The integral I am trying to solve is: (2kz/4(pi)(epsilon_0)*1/(z^2+x^2)^(3/2) dx. I know the answer is (2kz/4(pi)(epsilon_0)*(x/[z^2(z^2+x^2)^(1/2)] 2. Relevant equations I'm sure I have to set u=(z^2+x^2). This makes du = 2x. 3. The attempt at a solution I'm confused as to what to do now. The equation I'm integrating doesn't have an x in it anywhere. I don't think I can say du/2x=dx because I will have x's and u's in the integral, which is no good. However, I can't just ignore it. Also, how did that z^2 get in there on the bottom? z is a constant in this integration and since u = z^2+x^2, the z-term drops right out. I feel terribly lost.