The electrostatic force between two charges is given by
[tex]F= k_e \frac{q_1 q_2}{d^2} [/tex]
where ke is the Coulomb's constant, q1 and q2 are the two charges and d is the distance between the two charges.
We can see from the formula that the force is proportional to [tex] \frac{1}{d^2} [/tex]. This means that if we reduce the distance to one-ninth, i.e. the new distance is
[tex]d_{new} = \frac{1}{9}d [/tex]
then the force will scale as
[tex] \frac{1}{(d_{new})^2}= \frac{1}{( \frac{1}{9} )^2d^2}= \frac{81}{d^2} [/tex]
So, the new force will be 81 times stronger than the initial value, therefore the new force is
[tex]F_{new}=81 F=81 \cdot 3.0 N=243 N[/tex]
