Answer
$+23.06\ kJ/mol$
Work Step by Step
From the definition of heat capacity:
$155.5\ g\times 4.2\ J/g.K \times(18.6-16.2)\ K)=1567.4\ J$
Positive value because it's an endothermic process.
Number of moles of $NH_4NO_3$:
$5.44\ g\div 80.04\ g/mol=0.0680\ mol$
Enthalpy of dissolution:
$+1.567\ kJ/0.0680\ mol=+23.06\ kJ/mol$
