Albert Barnes on Acts 19:39

But if ye enquire. If you seek to determine any other matters than that pertaining to the alleged wrong which Demetrius has suffered in his business.

Other matters. Anything respecting public affairs; anything pertaining to the government and the worship of Diana.

In a lawful assembly. In an assembly convened, not by tumult and riot, but in conformity to law. This was a tumultuous assemblage, and it was proper in the public officer to demand that they should disperse; and that, if there were any public grievances to be remedied, it should be done in an assembly properly convened. It may be remarked here, that the original word rendered assembly, is that which is usually in the New Testament rendered church. εκκλησια. It is properly rendered by the word assembly--not denoting here a mixed or tumultuous assemblage, but one convened in the legal manner. The proper meaning of the word is, that which is called out. The church, the Christian assembly of the faithful, is made up of those who are called out from the world.

(3) "lawful assembly" "ordinary"