Explanation

(a) From Coulomb’s law the force of repulsion between similar charges ($q,q$) separated at a distance $r$ is given by
$F = \frac{1}{4\pi\varepsilon_{0}} \bullet \frac{q^{2}}{r^{2}}$
From the symmetry of the eight charges with respect to the centre of the cube, it follows that the force at the centre due to two opposite charges cancel in pairs. Hence, the net force at centre is zero.