A disk of radius a/4 having a uniformly distributed charge 6C is placed in the x-y 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 point 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=±a/2, y=±a/2, z=±a/2. The electric flux through this cubical surface is: