Given
Mass of each sphere, m=3.785 g=0.003785 kg
Distance between them , r=2.939 cm=0.02939 m
The acceleration is

To find
The magnitude of each charge.
Step-by-step explanation
Let magnitude of each charge be q
The force between two charges is

Again,

Comparing these two equations

Conclusion
The magnitude of charge on each sphere is 0.31mC