4Ca (ClO3) 2

What mistake did Carl make?

Cl03 is a polyatomic ion with a charge of - 1. So 2 of those brings that charge to - 2. Ca=+2. So Ca=+2 and (Cl03) 2 = - 2. This would balance out the equation. The final result would be Ca (ClO3) 2

He did not multiply the calcium atoms by the subscript 2.