Except as provided in section 9a, a metropolitan district charter passed pursuant to this act may be amended as provided in this section. An amendment may be proposed by the legislative body of the district on a 3/5 vote of the members or by an initiatory petition as provided in this act. If the amendment is proposed by the legislative body of the district, then the amendment shall be submitted to the electors of the district as provided in this act at the next regular election held in the district that occurs not less than 84 days after the proposal of the amendment. If the amendment is proposed by the initiatory petition as provided in this act, then the amendment shall be submitted to the electors of the district as provided in this act at the next regular election held in the district that occurs not less than 84 days after the filing of the petitions. The form in which the proposed amendment to a district charter shall be submitted on the ballot unless provided for in the initiatory petition shall be determined by resolution by the legislative body, and when provided for by the initiatory petition, the legislative body may add that explanatory matter as it considers advisable.