But if we inquire - 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 what is usually in the New Testament rendered church - ἐκκλησία ekklēsia. It is properly rendered by the word "assembly" - not denoting here "a mixed or tumultuous assemblage, but one called out, or convened in the legal manner." The proper meaning of the word is "what is called out." "The church, the Christian assembly ἐκκλησία ekklēsia of the faithful," is made up of those who are called out from the world.

But if ye inquire any thing concerning other matters - In which the safety of the state, or the national worship, is concerned, know that such a matter is not the business of the mob; it must be heard and determined in a lawful assembly, εν τη εννομῳ εκκλησιᾳ, one legally constituted, and properly authorized to hear and determine on the subject.

But if ye enquire any thing concerning other matters, it shall be determined in a (r) lawful assembly.
(r) He speaks of a lawful assembly not only to speak against the disordered tumult of the people, but also against all meeting and coming together which was not by order: for there were certain days appointed to call people together in.

But if ye inquire anything concerning other matters,.... Than what belongs to the craft and business of Demetrius, and the artificers:
it shall be determined in a lawful assembly; that is, called together according to law, and who have a right to hear, try, and judge causes, which such a confused lawless assembly as this in the theatre had not.

if ye inquire--"have any question."
concerning other matters--of a public nature.

In a lawful assembly - In such a regular assembly as has authority to judge of religious and political affairs.