The metropolitan council may at the same election called to authorize the performance of an additional function or at a special election called by the council after it has been authorized to perform any metropolitan function submit a proposition for the issuance of general obligation bonds for capital purposes as provided in RCW 35.58.450 or a proposition for the levy of a general tax for any authorized purpose for one year in such total dollar amount as the metropolitan council may determine and specify in such proposition. Any such proposition to be effective must be assented to by at least three-fifths of the persons voting thereon and the number of persons voting on such proposition shall constitute not less than forty percent of the total number of votes cast within the metropolitan area at the last preceding state general election. Any such proposition shall only be effective if the performance of the additional function shall be authorized at such election or shall have been authorized prior thereto.