An infinitely long solid cylinder of radius R has a uniform volume charge density  $\rho$. It has a spherical cavity of radius R/2  with its centre on the axis of the cylinder, as shown in the figure. The magnitude of the electric field at the point P which is at a distance 2R  from the axis of the cylinder, is given by the expression $\frac{23\rho R}{k{\in }_0}$  The value of k  is