§ 37. Disposition of real estate; franchises. No ordinance shall be passed making or authorizing a sale or lease of city real estate or of any franchise belonging to or under the control of the city except by vote of three-fourths of all the members of the common council. In case of a proposed sale or lease of real estate or of a franchise, the ordinance must provide for a disposition of the same at public auction to the highest bidder, under proper regulations as to the giving of security and after public notice to be published once each week for three weeks in the official paper or papers, provided, however, that where the real estate proposed to be sold or leased was acquired by the city pursuant to the provisions of § 72-m of the general municipal law for the purposes set forth therein, or where such real estate is owned by the city and devoted to such purposes, the ordinance may provide that such real estate shall be sold or leased to the highest responsible bidder upon sealed bids after notice thereof has been published in the same manner as hereinbefore set forth for sale or lease by public auction. Such notice shall contain a statement of the time and place where all bids received in pursuance thereof will be publicly opened and read, and pursuant to such notice, all bids received shall be publicly opened and read at the time and place so specified. A sale or lease of real estate or a franchise shall not be valid or take effect unless made as aforesaid and subsequently approved by a resolution of the board of estimate and apportionment or other governing body. No franchise shall be granted or be operated for a period longer than fifty years. The common council may, however, grant to the owner or lessees of an existing franchise, under which operations are being actually carried on, such additional rights or extensions in the street or streets in which the said franchise exists, upon such terms as the interest of the city may require, with or without an advertisement, as the common council may determine; provided, however, that no such grant shall be operative unless approved by the board of estimate and apportionment or other governing body, and also by the mayor.