Answer
$-68.9\ kJ/mol$
Work Step by Step
Number of moles of silver nitrate:
$250.\ mL\times 0.16\ M=40\ mmol$
Number of moles of NaCl:
$125.\ mL\times 0.32\ M=40\ mmol$
Both are in stoichiometric proportions.
From the definition of heat capacity:
$(250+125)\ mL\times 1.0\ g/mL \times 4.2\ J/g.K \times (22.90-21.15)=2756.25\ J$
With a negative sign since it's exothermic.
Per mol:
$-2756.25\ J/40\ mmol=-68.9\ kJ/mol$
