Hello, I am doing electrostatics at the moment and have difficulties to solve the following problem. Any hint that helps me to find the answer will be appreciated! "On every corner of a square are movable point charges with the charge of 'q'. Where does a fifth movable point charge have to be and what charge does it need to have so that the system is in equilibrium?" The answer is -0.957q and I do not know how to get that.