Call

Simply cancel out the common items, the ones that do not undergo chemical change in the original equation:

$2Fe^(3+) + cancel(6Cl^(-) + 6Na^(+)) + 3CO_3^(2-) rarr Fe_2(CO_3)_3(s)darr +cancel(6Na^(+) + 6Cl^(-))$

Are mass and charge balanced in the given equation? If they are not, you know that the equation cannot be accepted as a representation of reality. In other words these equations must obey