To better illustrate this problem, the knowledge of the charges of the ions is necessary.
Na2CO3 is formed when Sodium and Carbonate ions are combined. Sodium ion has a charge of +1 and Carbonate ions has a charge of -2. Remember the "crisscross" method when dealing with this problem.
`Na^(+1) + CO_3^-2 -> Na_2CO_3`
Same as with K.
`K^(+1) + CO_3^-2 -> K_2CO_3`
In the case for Calcium, it has a charge of +2. When you do the crisscross method, the subscript will have both 2 and it will be cancelled.
`Ca^(+2) + CO_3^-2 -> CaCO_3`
In CaCO3 ,
`` only one Ca required
valency here for Na and K are (+1) and CO3 is (-2)
so we need 2 Na and 2 Potassium molecules.