The Board of Aldermen of Duquesne shall provide for the manner in which such regulations and restrictions and the boundaries of such districts shall be determined, established and enforced and from time to time amended, supplemented or changed. However, no such regulation, restriction or boundary shall become effective until after a public hearing in relation thereto, at which parties in interest and citizens shall have an opportunity to be heard. At least fifteen (15) days' notice of the time and place of such hearing shall be published in an official paper or a paper of general circulation in the City.