Factory Reset

[nixonsherrie]

Is factory resetting an android samsung j3 enough to ensure there is no one monitoring the phone?

 

[mikedt]

Usually yes, provided it hasn't been rooted and modified, and any associated accounts haven't been compromised.

 

[lvt]

Yes as long as the operating system is original.

And as mentioned above, better update your Google account with a new and hard to guess password.