we know that it takes 2 moles of HNO3 to neutralize and each mole of Ba(OH)2 and the total number of moles of Ba(OH)2 to be neutralized is
23.410^(-3) litres 0.65 mol/litre
= 15.2110^(-3) moles
[ 215.2110^(-3)
= 30.4210^(-3) moles] of HNO3 in a volume of 42.5 ml
The molarity is 30.4210^(-3) moles / 42.510^(-3) litres
= 0.72 C
hope it helps