Where a protest against any such amendment, supplement or change is presented, at or before the public hearing duly signed and acknowledged by the owners or their agents, of 20% or more of either the area of the lots included in such proposed change or the land immediately adjacent to the rear thereof extending 100 feet therefrom or of those directly opposite thereto extending 100 feet from the street frontage of such opposite land, such amendment shall not become effective except by a favorable vote of all the members of the Board of Trustees.