The surface charge density on a nonconducting disk of radius \(R\) varies with the radius as \(\sigma(r)=c r\), where \(c\) is a positive constant \((c>0)\). Derive an expression for the electrostatic potential as a function of position \(x\) along an \(x\) axis that runs through the disk center and is perpendicular to the disk face, with the disk center at \(x=0\) on this axis.