Two point charges +2 C and + 6 C repel each other with a force of 12 N. If a charge of $-$ 4 C is given to
each of these charges, the force now is

Force,  $F=k\frac{q_1{q}_2}{r^2}\Rightarrow 12=k\frac{2\times 6}{r^2}$                 …(i)

When a charge of - 4C is given to each of these charges, then

$q_1=-2\mathrm{C},q_2=2\mathrm{C}$

and    $F\text{'}=k\frac{(-2)\left(2\right)}{r^2}$                     …(ii)

On dividing Eq. (ii) by Eq. (i), we get

$\frac{F\text{'}}{12}=\frac{-4}{12}$

$F\text{'}=-4 \mathrm{N}$

Here, negative sign indicates that force is attractive.