#### Answer

The initial acceleration of the 2Q mass is $a_0$.

#### Work Step by Step

The force that each one initially feels depends on the 2 charges, their signs, and their initial separation. By Newton’s third law, each one feels the same magnitude of force from the other.
An object’s acceleration is the net force on it, divided by its mass. Since the net forces on each charge are of the same magnitude, and they have equal mass, their accelerations have the same magnitude.