Particle a has twice the charge of nearby particle
b. compared to the force on particle a, the force on particle b is

The force between charged particles is calculated through the Coulomb's law,  
                                     F = kQ₁Q₂/d²
From the equation, it can be deduced that the force is linearly directly proportional to the charge. This means that when the charge is doubled, the force is likewise doubled. Therefore, the compared to force on b, the force on a is twice.