1. The problem statement, all variables and given/known data Here are the problem statement and the solution. I'm stuck at where the book suggests the formulas for the x and y coordinations (highlighted in yellow) of the third charge. Any explanations or proof on how they came to the conclusion for the third charge coordinations would be much appreciated. Is there an easier solution for this problem? The given solution: 2. Relevant equations x3=x2-rcosθ y3=y2-rsinθ 3. The attempt at a solution I think it must have something to do with the "point-slope" form of the equation of a straight line, but stuck in finding a reasonable proof.