Calculate the concentration of hydronium ions in each solution.

-->17.9mL of 0.175mol/L HNO3 added to 35.4mL of 0.0160mol/L Ca(OH)2

All you are doing is multiplying the number of Litres by the number of molecules in each Litre to get the number of molecules then multiplying by the number of ions in a molecule to get the number of ions. Then you cancel out the Hydronium and Hydroxide ions that combine to form water and divide by the number of Litres to get the number of ions per Litre. Have a shot at the second one yourself.