Rule # 1 indicates that the compound CaCl₂ should be selected and rule # 2 indicates that the number of chlorine atoms on the left side of the equation should be changed to 2. You do this by placing a 2 in front of the HCl molecule. Do not change the HCl molecule to HCl₂ because that is not a real compound. The above equation now looks like...

CaCO₃   +   2 HCl   ®   CaCl₂   +   H₂O   +   CO₂

After an initial step is completed, look over the equation to see if the change you made balanced the equation. In this case, we find that our task is complete and all the atoms are balanced on both sides of the reaction arrow.