A disk of radius ${a \over 4} having a uniformly distributed charge 6C is placed in the xy-plane with its centre at (-a/2, 0, 0). A rod of length a carrying a uniformly distributed charge 8C is placed on the x-axis from x = a/4 to x = 5a/4. Two points charges -7C and 3C are placed at (a/4, -a/4, 0) and (-3a/4, 3a/4, 0), respectively. Consider a cubical surface formed by six surfaces x=\pm a/2,y=\pm a/2,z=\pm a/2$. The electric flux through this cubical surface is