According to the Coulomb's electrostatic force(F) of attraction or repulsion between two charges is directly proportional to product of charges.

So $\mathrm{F} \propto \mathrm{q}_{1} \mathrm{q}_{2}$

The one of the charges should be negative.

a) IN the first case $q_1 = -3Q$ and $q_2 = 5Q$

Now $\mathrm{F}_1\alpha (3\mathrm{Q})(5\mathrm{Q})$

And in the second case when the two charges are touch each other then the common charge on each sphere is

Then $\mathrm{F}_2\alpha (\mathrm{Q})(\mathrm{Q})$

Then

Now $\mathrm{F}_2 = (1 / 15)\mathrm{F}_1$

b)

The nature of force is repulsive