The formula of carbonate of a metal M is M2CO3.
Valency of carbonate is 2
As here two metals are present,
So, the valency of metal, M is 1.
(a) Valency of iodide is 1.
So, the formula for iodide of metal M will be MI.
(b) Valency of nitrogen is 3.
So, the formula for nitride of metal M will be M3N.
(c) Valency of phosphate is 3.
So, the formula for the phosphate of metal M will be M3PO4.